I wonder if a plumber’s plug would provide a better end cap seal than the acrylic with o-ring. They are designed to operate at 120 psi pushing out. The seal would actually get better as you increased the depth.
1. pot/seal all end caps with wires going in. The opposite side would be a plumber’s plug.
The cap nests inside the pipe with a thick rubber ring. As you tighten the bold the rubber is compressed against the outer wall. One can get them so the metal disk would overhand the pipe, so that it cannot be pushed too far inside the pipe. See example photo below: