Do crabs have blindsight out of water?

A wizard can choose a crab as a familiar, and crabs have a blindsight of 30ft and are amphibian.

I can understand why crabs would have blindsight in water, as they can notice movement in the water and (apparently) changing salt levels to detect enemies. But it makes no sense for a normal crab to have blindsight out of water.

But from a purely mechanical standpoint, nothing is stopping me from having a crab familiar, and get 30ft blindsight by sharing senses with my familiar. Essentially having a constant "detect invisibility" seems a bit strong though for a 1st lvl spell.

Thoughts?