Updated 8/15/2021:
I applied plastic wood to the area at the bottom of the backwater. There's a sizeable crack between the plywood floor of the pond and the fascia. Untreated, that would definitely leak when I pour epoxy water in there.
Hopefully this will work. After it dries, I will paint and caulk the area.
My wife says to me (right after I do this), "Maybe you should have used Flex-Seal".
So, I'm thinking yeah, why not? I can just see their next commercial on TV...
"Phil Swift here for Flex-Seal."
"It's like a handi-man in a can and to prove it, we're going to seal up this pond on a model railroad"...
Well, maybe not.
George