Yes, the crab has blindsight out of water
There is nothing saying that the blindsight turns off out of water, so it stays active.
It's not unbalanced
The crab has 30ft blindsight, and 9 passive perception. Quite useful, but look at the other familiar options:
- Bat: Flying, 60ft blindsight, +1 perception, advantage on perception to hear
- Cat: +3 perception, 40ft movement, 30ft climb, advantage on perception to smell
- Hawk: +4 perception, 60ft fly, advantage on perception to see
- Octopus: +4 Stealth, advantage on stealth underwater
- Owl: 120ft darkvision, 60ft fly, doesn't provoke opportunity attacks, advantage on hearing and sight!
- Rat: advantage on perception to smell
- Raven: +3 perception, can mimic sound!
- Spider: Can use its web to sense location of creatures
- Weasel: +3 perception, advantage on hearing and smell!
Clearly crab isn't the strongest choice. The bat is basically a flying crab, that can blindsight twice as far and has advantage on hearing, and 2 better perception.
If you want to detect invisibility, remember that you can still detect an invisible creature with hearing, smell, or even sight by noticing signs of their presence. Your best bet would probably be the weasel, followed by the owl, bat, at, hawk, or rat.
A common tactic is to have an owl fly around above the party at all times, keeping an eye out and listening carefully for any threats. When there's a threat the owl can fly down to 100ft from you and telepathically tell you what it detected. It can do this indefinitely, even if the party is sleeping. This is a lot more strong than having 30ft around you to detect invisible creatures.