You'll at least need to know what group size you currently have installed, so the new battery you purchase will fit in your battery compartment or battery box. Group 24 or 27 are the most common sizes in wake boats from what I've seen. Chatt's suggestion is very overkill unless you tie up for hours on end and your sound system demands a large amount of power. Just about any group 24 marine battery has plenty of power to fire over a V8 inboard. Keep in mind if you don't have the old battery with you when you purchase a new one, you'll have to pay a core charge until you return the old one.
My Indmar engine manual says "Your engine uses a special 12-volt marine battery. If replacement is needed, use only a marine battery with minimum 650 cold cranking amps at 0 degrees F." I had group 24 Interstate Marine batteries that came in my boat from the dealer. They lasted 3 seasons and i replaced them with Walmart specials - Everstart Marine group 24's. They are $65 a piece and are doing great on their 2nd season of use. Academy Sports & Outdoors also has great prices on Marine batteries.