How do you use the boat? Start/stop all the time only or hang out with radio running? Aftermarket stereo, subs?
I've had a Blue OPTIMA Starting battery for years (8?) that is a bulletproof starting battery. For a ski boat that starts and stops all the time, and doesn't run accessories while off that have large loads for long periods, its been the best option.
On another boat, that often sits at the local sand bar with tunes running and big subwoofers, I use the Exide Dual Purpose AGM batteries. They take a beating with heavy loads and deep discharges and still start the 502ci engine. These have been WOW batteries for me, never had I such long discharge times and good recovery. On the same boat the OPTIMA Deep Cycle batteries did not last very long.