Golf cars - good way to go!
Years ago I installed golf carts and was poo-pooed because they weren't "marine" batteries. Well, I'm still on my second set since 1994 and it's been somewhere around 13 going on 14 years now.
They cary a lot of amp-hrs, won't break your back like a 4D or 8D, and they're cheap. Also, I've yet to put my spreaders in the water or do a roll-over.
As for needing a separate start battery, the answer to that question depends how bad do you need one. Shortly after we bought our boat I had a pair of dead batteries partly thanks to the dealer letting them go dead while showing the boat and, secondly, partly do to me not properly monitoring electrical consumption. Leaving the anchor light on was the last straw for them and by morning there was nothing left.
For some people, "once burned, twice learned." Needless to say, I learned from this experience and never have had a battery so low that it wouldn't start the engine.
Hearing that you have a pull-start outboard, and maybe even one with some kind of charging capability, means you're in a good position. And, being a trailer sailor you probably pick better weather to sail in so a solar panel or two might be in your future - another good charging source.
Having never taken auto shop in school I'd like to thank Bill for his comments about how to check batteries. I've noticed before that some cells don't seem to gas as much as others.
My golf carts don't have access to the individual cells except via the battery caps and I never thought about measuring voltage via the electrolyte. I'll give it a try but maybe put a piece of copper wire on the probe that goes into the electrolyte, though. Thanks, Bill, for your info.