Depends on what you are waxing. On my aluminum river boat, I use plain old car wax. On my fiberglass Penn Yan, I use 3M premium marine wax which produces a much better, longer lasting shine. I guess when it comes to waxing fiberglass, the phrase "you get what you pay for" really applies. The 3M products are really impressive as far as end results and what they claim they can do.
On the other hand, I would think that car wax would not hurt any boat, it just might not produce the best results.