Karl Timmermann wrote:

I am trying to finish my electric bilge pump for my kayak.

A Google search will turn up a herd of manufacturers of plastic check
valves and nearly all of them have a simple little diaphragm check valve
that will suit your needs.

If you want to prevent the water between the pump and the thru-hull from
draining back into the bilge you will need to mount the check valve
directly on the pump discharge fitting.

This isn't an electronics problem, it is a simple application of an
easily obtained mechanical component installed in the correct place.

Rick