Sounds like an I/O boat. I/O boats can and do go through starters like they are dirty socks... Why? Engines sit so low in the bilge, right in the stern where all the water collects, that they corrode away in no-time due to the super humid environment. On some of the GM based engines it's not uncommon for them to become submerged on a fairly regular basis.. If this is an I/O boat I would strongly urge you to focus on outboard powered boats, for numerous reasons....
The other possibility is undersized starter wiring, weak batteries or mechanics using cheap Chinese knock-off starters...