I'm no expert
I replaced the batteries on my 1996 281 last fall. I got 2, group 24 deep cycle batteries and use them for both house and starting. I have a Perkins 18 hp diesel and it doesn't take much to get it started so the I didn't see the point in having a dedicate starting battery. Based on how you use your boat, maybe separate batteries are justified. I have been told that the best way to get maximum performance from a battery bank is to replace the batteries at the same time with like batteries. That way you don't have one robbing power from the other. One other point and I may be wrong but I think the starting battery on my boat is the one under the sink and the house battery is the one in the engine compartment. Sounds counterintuitive I know.
