Joe: I'll have to cast a vote that the conduit was "pulled" out of the (probably) aluminum condulet. You said "floating dock"......someone possibly forgot to take that into consideration when they made-up the pipe. Movement, or possibly "contraction" due to the length of the run and the temp when it was installed can factor in also.
Think Good: Yes, there is PVC coated RGC for corrosive areas. It's a BEAR to work with!!
Sch 80 PVC, if otherwise suitable for the location is the preferred. That is, after checking the spec, plans, and assuring that it's otherwise approved for the intended install. And, you can't forget the expansion joints.
two dissimilar metals causes galvanic action. is the use of penetrox (or its equivalent) be used and comply with code. as to the installation the lazy electrician probably crossthreaded the pipe and after one thread caught left it that way. I agree that plastibond or ocal should have been used and the condulets cost a fortune.