Explosive gases.....
If you leave it in the engine compartment, you'll need to leave it open to the atmosphere, so that explosive gases do not build up in the hull of the ski. Might be a good idea to remove your ground cable too.
Myself, I think, if you have to use a maintainer through the riding season, your battery is crap and could leave you stranded. Or if it's new but being run down, then your charging system isn't suppling the 14 vdc that it's suppose to.
Check your charging system with an in-expensive voltmeter. Your battery, replace it if you have to charge it through the summer. IF you take if to "Auto-Zone", they will test it to see how many cells are shot.....