If I was getting a fun and fast boat for the long boating season in Florida, I'd put three things at the top of my list:
1) Staggered engines so its easier to service.
2) Carburetors, as salt water demonizes electronic engine sensors, controls, and wiring
A distant third would be
3) Closed cooling system.
I don't think this is so important as you would think, because the key thing that dissolves is the exhaust manifolds, and everybody puts the raw water through them. The blocks and drives seem pretty immune to salt water.
And an even more distant:
4) An efficient stepped bottom.
As fuel costs continue to go up, the amount of fuel you burn will have a bigger impact on how often you go out. Its already happening to many of us. I do consider, say, flying to Hawaii over taking the boat to Catalina at these prices, and when they double again...