Since you have docks on the lake, I assume that you frequently have people on or in the water.

I am not an electrician, but I would NOT want 110V AC current running through that water. You absolutely need a certified electrician for this project. He may say your plan is a code violation, and that kind of answers your question.

Another option would be to install a power converter at your dock to change the AC to 12V DC current. Your inlaws could then run 12V lights and the battery charger at their dock. I think that might(?) be a safe alternative. I do not know if my plan would work though, since I don't know about the voltage drop for 12V DC over a 200' wiring run. All of my wire sizing calculators are only for AC electricity. I would still get an estimate from an electrician for my alternative plan, and if it is stupid he will probably immediately tell you so.