Drag a meter through the vicinity, find the likely culprit, sink his vessel (hope it's not an old Seacraft)and then give him a highlighted copy of Calder's big book ... that'll learn him
Check out Chapter 5 page 211-213 in Calder's 3rd edition of Boatowners Mechanical and Electrical Manual, there he gives a 10 point plan to protect your stuff from the less concerned. Create a low resistance path to your battery and then bond everything to that good ground. You have a big challenge, you definitely will need to bond that drive shaft to a good ground. Oh, yeah, I have a nice hole saw and auger bit collection should you prefer that route.
Good Luck!