HCA: Hermit Crab Association

I have 2 6x6x6 SW & FW pools each with air stones, but pretty much the only time any of the crabs actually submerge is when they accidentally fall in. Now when I first put them in 6 mns ago or so, I placed some of the crabs in them and they stayed under for hours, plainly enjoying the bubbles.

I change the water regularly, and each has a ramp that is easy to climb.

When they want to drink they hang over the side and/or sometimes go down the ramp until the water meets the ramp and stop there.

Any ideas? Any help with what I can do to get them swimming?

They usually do it when no one is watching, lol. I have PP's that like to hug the bubblers and some that hang off the edge. Provide them with the deep pools and they will eventually figure it out if they haven't already.
Honestly they probably are swimming while you're asleep or no one is looking.

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A couple of my PPs will go in the water at any time while the other two tend to get wet because they've fallen in from one of the over-hanging vines. But, as @soilentgringa said, they swim when you're not looking, in the wee hours. How else does all that poop and sand get in there ?