I usually install the pool so that at least the outer rim of the pool is about half an inch to an inch higher than the surrounding substrate. The crabs can still climb over to reach the pool or just get out.
Here's an image of one of my old tank configurations where you can see the rim of the pool (an old tupperware container) still above the sand line to the right of the tank in this photo:
Ideally, what you have right now is what you may find in nature, but in a tank in the long run, it's a real pain to maintain, with the crab's habits plus the capillary action of sand will draw water from the pool and into the surrounding substrate, causing bigger problems.
You may need to reinstall to raise your current pool a bit higher than the sand, and perhaps consider buying and installing one of those terrarium water containers sold in the petshops - or rig one from existing containers you can find in your home.
And for maintenance, I just make sure that when the crabs start digging, no clumps of sand form around the pool rims so that none of the sand touches the water - I just clear it out with a spoon or something when the crabs start making sand mounds around the pool.