There should be a MSDS for the CPES that you can check for toxicity. I would assume it is inert when cured. It wouldn't be very useful if water washed it away.
Linseed oil isn't considered toxic in normal use and I would consider this normal use. It will find it's way into the pond and be decomposed like all organic material. Just like all plant oils are (algae, pond plants and the pine boards) washed into the pond and decomposed. The amount of organic matter added to the pond would be very tiny compared to fish food and decomposing plant matter.
Polyester resin is very toxic to fish and people, flammable, and other bad stuff. The catalyst MEK is even worse. Cured polyester resin is basically inert and not toxic. Fiberglass, as a dust is very toxic to fish and people. Fiberglass in mat form and encased in cured polyester resin is basically inert and not toxic. So context is important.
