My Yanmar 3GM30F started just fine with a group 24 deep cycle battery. When I moved the start battery closer to the engine went to a group 27 deep cycle just to have more reserve should the primary golf cart bank fail. Probably should have stayed with a group 24 as I'm not sure I can get the group 24 battery out without distantling the galley because of the additional weight.

If you have a larger engine follow the manufacturers spec's as you can burn out the starter if not enough amps. Know of one cruise that ended prematurely because of to small battery(s) leaving the boat with a dead engine one time to often.