Mark,

On my very flat bottom very hard riding X55, I too learned this lesson. Years back I went though 3 high CCA batteries (around 1000) in 4 years then, I learned what was going on. With all the lead plates in the high CCA batteries taking up the space that support was less so they were breaking and shorting. Yes, bigger is not always better. 

I have found over the years that the batteries with higher cranking amps also have more lead plates inside.  Good for extra power but in a boat those larger plates can start banging around and short out.   The lesser power batteries don't seem to have that issue.   I would still want something with at least 650 CCA so you have enough power on hand.  The old flat bottom Nautiques can be rough when the water is not nice, it is hard on your battery.

James,

Why the "marine" battery? Do you actually need the terminal studs and the lugs? The word "marine" on anything automatically increases the price! 

It'll work just fine and it's pretty easy to find a 25% discount coupon for a computer order from Advance Auto. It might be too tall unless you modify the battery box cover though.

Order and pay with PayPal, charge card etc, walk in the store and they hand you the battery  Check the height of the battery and your battery compartment. It might be a little too tall for the compartment.

Yes, automotive starting is a great choice. The cost of lead has gone up so they are getting pricey.