Epoxy resins are generally reasonably waterproof. The main problem is when applied over polyester resins is that they trap water in the laminate which wants to enter due to osmotic pressure.
An Epoxy barrier coat seems a sensible option for new hulls, but this treatment has not enough history to project future outcomes.
Epoxy treatment on a polyester resin boat that has already been immersed may do more damage than good by trapping moisture.
Why donít more builders use epoxy for the lay-up? . The additional cost would be small compared to total for the total cost of the boat. Oyster HR etc lets see at least an epoxy option.