A wizard can choose a crab as a familiar with the find 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 getting 30 feet of blindsight by sharing senses with my familiar. Essentially having a constant "detect invisibility" seems a bit strong, though, for a 1st-level spell.
Is this correct? Do crabs have blindsight out of water?
Best Answer
Yes, the crab has blindsight out of water
The statblock for the Crab adds no additional qualifiers to its senses, so the Crab has blindsight both in and out of water.
There are lots of things in DnD that make no sense. A mule can ride a phantom steed, for example. But it doesn't ruin the experience, as the great thing about DnD is that you change the game to whatever makes for the most fun experience. If you want a crab familiar that's blind out of water because it improves your immersion, then it's absolutely your right, with DM approval, to modify the statblock accordingly.
In terms of balance
It shouldn't be a concern. If you want to see this as an alternative way of casting See Invisibility, consider that a Crab can only see out to 30ft, while a PC casting See Invisibility can see out to their full range for the duration of the spell (1 hour, no concentration required). Additionally, it requires a full Action to switch to the familiar's perspective, meaning that the combat utility of the Crab familiar's blindsight is essentially null.
Additionally, a crab has a passive perception of 9, meaning that any reasonably stealthy invisible opponent will have no trouble defeating it with an average Dexterity (Stealth) roll, so its utility as a sentry is similarly poor.