I think you could do it without too much trouble. Pumps probably don't do well with sand, but if the pump inlet is not directly on the bottom and too near the waterfall/sand, the sand will settle before reaching the pump and all should be well.

I assume ambient water temperatures are tolerable!? Your specific design would require a waterfall that you can get your legs into but on a standing surface that doesn't create too much splashing whether washing in it or not. If splashing is serious, you might lose too much water although you can always add more. But you can't see to check it. The normal design is a big hole partially filled with one or more porous boxes similar to (or even) milk crates. These 'crates' allow rocks and you to be supported above them while they contain excess water and the pump so that there is always more than enough water in the tank AND the falls, tubing and spillways. Running the pump dry is usually bad or real bad.

Remember, in your case, at least the one part of the tank with the pump must be away from the turbulent sand/water area. Perhaps you could use two crates with a baffle between them that prevents the first crate water directly reaching the second except around the edges and mainly the top. Actually, this may all be overkill. I imagine that a sponge pump inlet filter would solve almost everything as long as there is some distance between sand and pump.