Yes crabsnakes can do damage to your seamoth. try to stay away from the jellyshrooms they live in and they should leave you alone.

Do they hold on to it?

Your seamoth? no you, kinda. they will jump out of the jellyshrooms and ambush you, if you are outside your seamoth.

They can damage the Seamoth, and if you are outside it, they can grab onto you and gnaw on your health. However, they actually are not as deadly as some other creatures. I believe Bone Sharks do more damage.

Just leave the crab snakes alone and they should too for you. Unless you want to kill them, I have done that with a knife and stun rifle.

Ye