I'd read the sealant label. If Life Caulk doesn't work with any "plastic" then you couldn't use it on a fiberglass hull, and I doubt it would have survived this long in the market if that was true. I think Marelon is more like the fiberglass material your boat is made of.

One tip -- don't over-tighten the Marelon mushroom thru-hull fitting. It's possible to crack it where the threaded portion meets the mushroom shape, especially if your hull is a bit thicker on one side of the hole than the other.