Sealabs pose novel engineering challenges, could provide experiments in sustainable enclosed habitats, wouldn't risk microgravity or radiation, but would risk high pressures.

They'd also help species survivability in the event of some extinction-level asteroids, climate change, or interstellar radiation.

I think it's an even money bet whether it'd be easier to build a comfortable habitat for x people underwater or in space (for most definitions of comfort, x, and space).