I have used Boss Sealant to floor seal fiberglass steps in my pool and around my shower stall. I've had no luck with other sealants (I've tried many other sealants including 100% silicone). So far, so good. Easy to use, dries quickly and is durable.
Pros
Designed for bonding ceramic tile to many substrates including acrylic, fiberglass, ABS, PVC, wood and masonry