I am in need for a new starter battery, the one I have been using was a 24SM-4, 700 CCA, 860 MCA with 125 minutes reserve.

I suspect this is the original battery.

One battery place suggested a 550 CCA, 650 MCA with 90 minutes reserve for my boat (180 Sea Ray with 3.0L). Cost is $55.

Question, how much lower than 550 CCA can I go? I saw a battery at Wally that is rated for 500 CCA and 625 MCA for less money. Cost is $46

Some may disagree with me, but I say buy the cheapest battery you can find, it'll work just fine. The main reason you'd need more amps than the cheapest battery is for turning over an engine at below zero temps...not much of an issue with your boat!

The only other thing you'll be giving up is a few minutes of "key on" time running just the stereo or some other device that uses the starter battery. And even that is minimal.

FR may be right in principal, but I'd go with the manufacturers specs. I know that the Volvo in my Four Winns has specific battery requirements (likely related to EFI or the ECM) and warns against the use of batteries below those thresholds.

I'll dig out the manual and see what I can find. I understand the need for CCA but in a boat maybe it should be changed to WCA as I am not boating if it is cold.

As far are "key on" time, we don't even use the radio. In fact, I haven't used the cassette eiter. A few times I may turn it on while at a no wake speed and my wife turns it off. I guess she likes to hear the water lap against the hull. I don't blame her.

Do you have a 4.3L V6 or which engine? FI or Carb?

I agree. At least meet the minimum that your engine requires. Not necessarily as big that came your boat though.

3.0L carb

I'd take the Walmart option.

I dug out the manual (Mercury MerCruiser operation, maintenance and warranty manual:
minimum recommended battery rating is 375 CCA 475 MCA and 90 min.

It looks like the Wally world battery is more than adequate.

The battery is purchased. It costs an extra $9 if you don't bring in your old battery. Mine came with a 12 month warranty.