I can tell you from personal experience that your problem could also be corrosion at your key switch from salt spray dripping into it. Clean it too! Go to battery posts and battery selector and clean and resodder lugs. If you have access to torch, "dump" the soder into the cleaned connectors. Clean excess flux using brake cleaner spray. Crimp on connectors will fail after proloned exposure to salt H2O. Of course, make sure fuel is turned off and FX is near by before resoddering with tourch. I have welder torch with small brazing tip. It works great. Having a buddy near by would be conforting in case you mess up. YOU WILL! Removing manifold is a great idea. I have dual 5.7 s and I have been able to inspect (upside down on my ****** that one starter has rusted surface but motors all seem to be well painted. I suspect that who ever owned boat befor me, replaced started when new engine was put in back in 2010, but they failed to paint new starter. Removing manifold will allow YOU access to concentrate heat onto specific starter bolt heads. Then, use (after heated part cooled) 6 point box, not 12 point, since 12 point box wrench or socket may strip head. Once out, resodder wire lug at solenoid/bendix end. Test solenoid/bendix and then starter. You may have water intrusion inside starter from some sort of "drip" line inadvertnetly created by boat's design. It wasn't planed. It just happens. I have them too! You may be able to clean inside starter, too. I was able to salvage a Surflow washdown motor just by cleaning inside. Starter bendix may not be salavaged. Go back with marine grade starter. Or have it rebuild by a professional, if simple cleaning will not fix it. Starter is usually the lowests point in bilge and would be first component exosed to gas fumes, if something else fails related to gas. Go back with anti sease lube on anly bolt threads, or just use grease on threads. This is my 2cents.. Good luck!