I am thinking Dolphins and sonar, loud sharp noises that travel far even underwater. Maybe clicking noises with their tongues, maybe popping noises made by pursing lips and blowing air.
If it is what I am suspecting and the reason they are called 'crab' people is because they have distinctive claws of some kind then definitely include snaping their claws at least for emphasis. If they have human hands as well as claws you can have an amusing side note where a foreigner snaps with their fingers in an attempt to communicate and the crab people(s) are shocked because they only use their claws to make the louder noises so they didn't know finger snapping was possible.
Also if like crabs they have more then 2 pairs of limbs and some of them have fingers, you could always go for sign language. The crab people could also have antennae which could lend a hand signaling wise. You could also have the crab people make like bees/ants and dance to communicate, especially if they have pointy pod things for feet then crabs tap dancing to tell a story would be pretty interesting.
Depending on how alien and or crab like you want the crab people to be having the ability from other underwater creatures like color changing and light flashing could be an option (the language could even be mostly silent between crabmen).
You also haven't mentioned if magic is a factor, you could always explain talking underwater clearly/unimpeded away with special spells or abilities like having the crabs all share a hive mind or a psychic connection to a crab mind internet.
