If i may chime in here:
I did major research on epoxy's and resins... I came to the conclusion that the fiberglass resin is not what I wanted to use (esp. if it is being done indoors, where venting the gases could be a problem, as it really is toxic compared to most 2 part epoxy resins). The biggest issue is that it has to be 100% fully cured no matter what it is otherwise it will break down in saltwater if it is not. This means that no matter what is used, the mixing procedure is a vital componet to a full successful cure.
However even though it may have worked, I could not find out from anywhere or anyone if it was inert after it was cured fully, so I decided against it and took it back to the store.
I came across many of articles in my search that suggested that using fiberglass resin (polyester resin) would be ok to use, but the styrene monomer that is used as a hardener could be a major problem if it does not cure 100%, although the same problem exist's with almost any 2 part system... as it has to be cured fully before it is safe to submerge in saltwater.
Most aquarium ornaments and many ponds are constructed with fiberglass resin (polyester resin), but they are left to cure for many months (at least 30 days).
After dsearching various resources on the web, I came across a product called MAX ACR. it is sold in 1 1/2 gallon and has a mix ratio of 2:1 with a 3 day 90% cure and 7 day 100% cure with a 60 minute working time. This stuff is marketed for building large public aquariums and is sold as fish & aquarium safe. I searched ebay for "Aquarium Safe Epoxy" and it was top of the list... This stuff is worth the money!!!! It works awesome esp. indoors because it is virtually oderless! This time of year, that is a very nice bonus unless your on the coast somewhere and it doesnt matter!
To sum it up, I would not use fiberglass resin personally, although it may be "safe" utimatley, the short working time (you only get 10-15 mins of working time with that stuff) the high oder, and the lack of support and info all steered me into going with something else.
Hope this helps!