Lots of different species of crab. Some are better swimmers, some are better walkers. My advice: give your crabs whatever speed best suits their role in the encounter.
The timing of this discovery is rather fortuitous.
I'm a player in a starting campaign, set in a sort of "place at the end of time" locale. My character is a space probe - an autognome warlock, but with all their powers being robot-based. For example, a familiar is a small drone. EB is a death ray. etc etc. As they level up, they repair and upgrade themselves. The patron is the builder of the probe - and the PC doesn't know who built it, or why, but knows his primary mission is to explore.
His subclass is fathomless - he's built to explore water worlds. This means he has a swim speed. And now I know how it works
(why not an artificer? another player is playing one, and there was a need for a "face" in the party)
Lots of different species of crab. Some are better swimmers, some are better walkers. My advice: give your crabs whatever speed best suits their role in the encounter.