That my Diesel truck that sits in the same garage as the boat , and often sits for months at a time without being used original batteries lasted 10 years, The replacement batteries are 8 years old and running strong.
My 2011 Camaro that sits in the same garage as the boat is still on it's original Battery(although it struggled with its first start this year).
Both of those vehicles are shut off, and covered with no battery shut off over their winter hibernation.
The boat on the other hand has battery shutoffs, so in theory no battery drain. Hooked up a charger to the number two battery the other night planning on a first outing last Friday, that was dashed due to High winds.
Hooked up the boat, pulled it into the driveway and went to check if she'll start at the lake this afternoon. No reaction when I set the key to run and a big Fat Zero volt reading, either or both batteries. Charger is on the batteries that were bought in June of 2020, hoping I can get them charged enough for the latest first outing since getting into the boating life.
Boat batteries last less than two years on average. WTH?

That is crazy. Just checked ours & its 7 years old going strong.

second time dead batteries have delayed the start of boating for us.
Left it on the 50 amp charger for about 2 hours today and only got a click with no turn over.
Jump started the boat with the truck and let it idle for about 15 minutes. Fired right up and ran smooth on the hose. Shut her down, and put everything away then tried to start it again. Cranked but not enough voltage to run the computer to fire the injectors. Back into the garage she went. On a two amp charge for the weekend then another attempt on Monday when I'm off work again.

I am 90% sure that your batteries are suffering from sulfation. Add a two bank onboard charger or, better yet, a single bank onboard charger and an ACR, and plug the charger in when you are done boating for the day. The typical marine engine alternator is barely adequate to re-charge a single battery while providing power for the boat underway. If you spend any time at anchor with any meaningful power draw, I bet you are putting the boat away with a battery that's well below fully charged, and that causes sulfation which causes the exact symptoms you have.

Many battery chargers have an "equalization" or "conditioning" cycle. This may fix the sulfation and it would be worth getting a charger like that since the option likely costs less than two new batteries. I have a Promariner charger on my boat and it's fantastic. It fixed my 6 sulfated house batteries and saved me a lot of money.

Do the batteries have any warranty left? Either way I think it might be time to replace. Not sure i'd have the confidence in them on the water.

No warranty left as they are now two years old.
Wondering if part of the problem is my battery charger.
I had an older Sears 6v/12 Volt battery charger trickle charger with a Starting jump mode. Was a great charger until it was left on a neighbors car overnight when it got rained on an failed shortly thereafter.
Did the tragic mistake of replacing it with a Diehard charger while sears was on it's downslide and quality of products was tanking. This charger shows the needle at full charge even with the 2/10 amp option and the charger gets hot, hums strongly, and the Green Charging light is on steady even after leaving plugged in overnight.
Just tried to start it in the garage and while it now cranks strongly, no fire. The kill switch is in place, I can't find any circuit breakers popped and I've verified the main circuit breaker is in. All connections I can find are fully engaged, but I'm not hearing the fuel pump, but that could be my memory as to whether I can normally hear it on first turning the key to run. All other normal power up items seem to work as I recall
When I jumped it the other day she fired right up and idled great, now nothing.

If it cranks strongly, it's not the battery that's keeping it from firing. What engine do you have?

330 HP 350 inboard. NorthStar injection.
The aggravating part is that it initially idled great when I jump started it. Let it idle with no problems to charge the batteries more.
When trying to restart it after putting everything away she initially tried to fire, but just wouldn't do it.
This morning there is NO evidence of firing, and pressing the Schroeder valve on the injection system there is no fuel coming out.

Threw in the towel and towed it to the Mastercraft Dealer. They think they may actually only need a couple days to troubleshoot and fix. Might get it back tomorrow afternoon, but I refuse to go to the lake on a Holiday like the 4th of July, which is my next day off after tomorrow.
But boy was that a depressing visit. Cheapest boat on the lot was listed at $176K, with most of them in the $240-$300K range and one approaching $400K. In the last year they sold over 120 boats with most of them being cash sales.
I'm in the wrong career.

How annoying. Hopefully it is a quick & easy fix.

Sure sounds like fuel delivery. If so, could be a relay, fuse, loose connection, broken wire, fuel pump, filter, or collapsed/damaged/cut fuel line. I think most marine engines build fuel pressure regardless of the engine turning, so I suspect it's not a crank position sensor. Good luck.

They Diagnosed it as a failed fuel pump, so less than 24 hours after pulling the boat out of the garage to get it fixed she's back and supposedly fixed. We'll take her out next Tuesday and check it out.
Nice quick work by Utah Watersports, and while if I knew how to troubleshoot it the repair was easy enough, but avoiding the aggravation and sweeting it out in the 100 degree weather we're having my time was better spent smoking some beef ribs and pork Belly burnt ends.
The fuel pump has been redesigned so they needed a new harness, and while it was there I had them replace the impeller since It hadn't been changed in a couple years.

Break out another thousand has been inflated, but hopefully the batteries are good through this year. Ordered a trickle charger to hook up and hopefully prevent another bad, late start next time.

Good to hear you are back on the water.

That's great news. Glad you got the charger, it will save you many a lot of money if you keep the boat a few years.

If the batteries are bad, consider converting to a dedicated starting battery and a deep cycle house battery with an ML-ACR to combine and isolate them. "Dual purpose starting/deep cycle" batteries are really just starting batteries with a different sticker on them and they don't handle deep cycles very well. The ML-ACR will give you the option of manually combining the batteries from the helm for a starting boost if needed, and otherwise will ensure that the start battery gets charging priority. When the starting battery reaches absorption phase, the ML-ACR will combine both batteries so that the house battery gets charged automatically. The ML-ACR works with any charging source, alternator, trickle charger, etc.

I just replaced my fuel pump. Good God. I feel I have been violated at $1400 just for the fuel pump. The local Volvo engine dealer said the pump was on a backlog and wont be delivered until Jan next year. I found one 100 miles away and delivered to the repair shop.
I replaced the pump 10 years ago. They said that is a good run on those pumps.

Seems weird to me that fuel pump was working & gave up the ghost all of a sudden without any signs of going bad. Can't help but wonder if it was somehow effected by the battery situation?

That is exactly how my fuel pump went out. Boat was running fine one day and the next morning it started and we got a hundreds up the channel and down it came. Able to kick it over a couple of times tp get headway back to my dock. Had to swim 50 feet to get my Jetski to tow it back to the dock.

Same thing 10 years ran good and then dead.

I think you're pretty close to what happened. GUESSING that since I didn't get the batteries fully charged when I jump started and only ran the boat at Idle for 15 minutes the alternator was having to both charge the batteries and run the fuel pump so the pump wasn't getting enough power and caused a 20 year old pump to fail. Had a similar situation years ago on a plane that we could "downspeed" the engines to 69% on the ground instead of the normal 100%. At that low RPM the generators were supposed to kick off line and busses were transferred to another power source. One of the Relays failed and held the bus to the downspeed engine, which caused an under-frequency situation which overheated and caused some systems under the flight deck to start smoking.

Glad it happened in the driveway instead of at the lake. Just talked with one of my neighbors tonight who has the same model of boat, just two years older and he had the fuel pump fail while at Lake Powell miles from the nearest marina. Fortunately they had a house boat and other toys, so they could tow that boat back to the marina and get parts shipped in.
All told I'm felling luckily that I found it where I did and that the dealer could get it fixed in one afternoon. Didn't verify she ran when I picked he up, so I'll see if she starts tomorrow we'll get the latest start ever to a boating season on Tuesday.

True very lucky to be able to get parts these days.

Batteries-- wish I could get the service from all batteries that I'm getting from my truck-- bought new in April 2002, end of August 2013 while parked on the launch ramp after launching the Chap, when I went to start to head back, the 11 year old battery just went BANG-- split the side of it out. Got a ride to Walmart, got a new battery, and THAT battery, almost 9 years later, is still starting the truck like new... Awhile back the battery in our '04 Chebby Venture went bad. For kicks I bought one of Walmart's 1-year batteries. I think it lasted about 369 days and just plain gave up out of nowhere... go figure!

As to fuel pumps-- high pressure pumps seem to suffer from either the impeller failing (coming loose for whatever reason from the motor shaft or just plain breaking), or the motor just dies, for whatever reason. Of the many fuel pumps I've had go bad, NONE of them gave any warning...

I've used one of the smaller Battery Tender brand trickle chargers for quite a number of years now and it does seem to have extended the life of my batteries vs. before using it. I only use it during the off season.