That looks really convenient in case I either need to lose my valuables or else take someone else's in one go.

I will say that it looks well made. They did a good job.

this is probably more for things that people won't bother stealing. I certainly wouldn't be trusting my phone to it.

I'd also be worried about it getting covered and misplaced/lost. piles of sand look much like another.

Could be a fun geocaching thingamabob, though.

Edit:
I'd also suggest sticking a silicone o-ring on the lid. you can make them pretty easily using fish tank air-hose tubing. Pump it full of calk. If you want a solid ring, let it cure, then blow it out with an air-blower nozzle on an air compressor. If you want a hollow ring, which for this would be best, I think; blow out the tubing before it sets. then blow it out again. cut the ends on a bias and then glue them with more calk.

For this, you'd never know when a kid comes running over with a giant pale of water.

Good call. I forgot about geocaching.

Also, thanks for the technical tips.