You get them from killing the giant crabs. Some of them will drop a shell when it dies, collect three and take them to the ship guy, he will then create two helmets and a breather which you can buy off him. It seems that the crab in the stout mines always drops a shell when it is killed.

Where are the shells that shipyard guy asks for ???
I guess he makes scuba gear of them ?
I read in the forum that someone found them in the stout mines. please tell me it aint so.