I planning on doing a display tank with a DIY waterfall and lagoon/pond at the bottom using a filter/pump.

I'm going to use great stuff minimal expanding foam to make the rock work, and Rustoleum American accents stone finish to add color and realism. I'm also going to paint the inside of the plywood enclosure with the same stuff with varying colors to make it more visually appealing.

My question is, what sealant is safe to use for constant submersion and temps typical of a snake enclosure that doesn't look hideous?