Inboard or outboard?
Yes a boat (or motorhome, etc) Can be considered a 2nd home. It must have a place to sleep, a place to go to the bathroom (who knows...a bucket might work) and a place to cook. that is the requirements, but you can generally only write off the interest on the Mortgage for the boat. thats it (unless you have it as a business...but then that a whole ''nuther can o worms)
BUT if you use a home equity loan there is no reason to set it up as a second home becaus you are already writing off the interest on the loan to get it.