Our motorhome, the SkipperLiner, and the Party Cruiser all qualify as 'second homes'. The motorhome is our 'tax advantage' unit, but the only actual tax advantage is being able to deduct the loan interest. (and it's the only one we owe money on). Property and other taxes aren't deductible on a federal level on any of them, but all taxes and fees are deductible on our state taxes.
If you charter/rent/lease/charge someone to use your boat, then it becomes a business, and you'll of course need a biz license. As business equipment, virtually every $$ you spend on it is deductible. If your business shows a net gain, all of it can be spent on the boat and deducted, and you'll have no extra income or taxes to pay. But if you have a net business loss, you won't be able to deduct the loss from your other personal income. Losses can only be applied against the next year's biz income.
I'm sure there's clever loopholes available, but I'll never know about 'em..
1988 SkipperLiner 53x14
2002 Regal 2860 Commodore 28x10
1995 Sun Tracker 32' Party Cruiser
1987 Sea Ray 21' Midcabin