you might only be getting yourself into more problems. It would depend on what you would want to use it for, but if you're considering a back-up starting battery, You have to consider cranking amps. If you're talking about one to maybe run a trolling motor, it's probably going to be the same thing, unless it would be for a very short distance. If it would be for all applications, ( starting, all electronics, trolling motor, etc) then nothing short of a full service marine battery with a high amperage rating should be considered. We have two heavy duty marine batteries that are just for the anchor and trolling motor. The boat motor died, and we used the trolling motor to get us close to "home". We were about 5 mi from the dock. Both batteries were fully charged. We made it about three of those miles before the batteries died. Trolling motors take a large chunk of power. Lots of thing to consider too: How big is the boat? How heavy? How much load ( people , motor , gear?) If a spare battery is really needed you should go with the largest amperage/physical size you can fit into whatever space you have for it. Also remember, if it's a small boat, that battery might add another 50 to 100 lbs. That's something to consider if your load is close to or at the capacity rating.
I carry one of those jump start battery packs. Most likely won't get you very far with trolling motor use,but will start your gas motor if your start battery fails. Completely useless if you have 24v trolling system.