There's no maintenance for speakers, per se. I would keep them clean. If they have butyl rubber surrounds, you could also treat the rubber, but I've never found it necessary. One thing you can do to make them last longer is to let them "warm up" when you first turn them on. Like any moving mechanical structure, they will sound better and last longer if you warm them up by keeping the volume at a low to moderate level for the first 15-30 minutes.
Cool thanks. The boat is stored in the garage and even though the stereo can get loud we don't usually turn it up that much. Actually the loudest we usually go is to be able to hear it while underway but I'll keep the 15-30 minutes in mind.