Same thing happened to me. But I don't think i had any saltwater get to the wood.(could be wrong)
My thought was that i must have sanded the wood to a different grit and the epoxy absorbed deeper
into he wood thus a darker color.

It's been a year, the patched area is still as dark as ever.

: Heya John. Yes, salt water. Glad you asked, I just knew I'd forget
: some little tidbit of important information....