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).
We'd do better to build out some element of human society underground, if interstellar radiation is our fear.
Actually, water is very good at stopping radiation, so if there was a massive gamma ray burst that killed all surface life on Earth, some small groups of humans would survive: deployed submarine crews.