Mothers automotive wax/cleaner is the best I've ever used, aside from the old Texaco stuff you don't see anymore.. About anything will be fine but the mothers wax/cleaner is great stuff and gives the best results.
I'm a true believer in removing all the lake scum before sealing the gel coat using wax, for that I've been using liquid toilet bowl cleaner called "The Works", brush it on the stains using a long-handled car-wash/RV brush and give it at least 5 minutes to work.