I recently ordered adhesive to replace some tiles that had fallen off my fiberglass pool from fiberglassswimmingpooltiling.com. The adhesive they sent me was just a silicone sealant made by DOW. It said right on the back that it was not recommended for underwater use. They were unresponsive when I emailed them about it.

I have found a couple of products that seem to be specifically made for this application. One is made by Novagard and the other by Boss.