There are two approaches to this question. One is the your engine
requires very little starting current
gas engine), so you don't NEED anything but a Group 24 starting battery. However, a C&C
33 should have a couple of quality batteries, so I would push the current
owner into buying
a pair of Group 27 deep cycle batteries, probably from West Marine
since they are made by East Penn, and East Penn makes damned good batteries.
Don't worry about getting deep cycle batteries. They will crank your engine
just fine, and be better for all other uses.