Wouldn't it make more sense if they just migrated to a tunnel network that was underground but had sea access. As in tunnels that terminate at caves and those caves have pools in them with flooded tunnels leading out to the sea.
Then perhaps your people could develop something like a breathing helmet made of leather sealed with pitch or resin or similar allowing them to move through the tunnels. They wont be able to see of course but then without a waterproof light source things are going to be pretty dark in those underwater tunnels anyway.
Over time the tunnels have ropes attached to them so that people can pull themselves to where they need to be or even pulleys so that things can be transported.
They could have breathing helmets placed in various places for emergency use including for people diving from the sea surface to use to escape into the tunnel network. If the helmets are tethered with the open end down then the air should remain inside.
Such a civilisation could engage in piracy along the coast. Sneak onto ships at night, steal what they need then dive overboard with their loot, escaping by diving down to the caves. The local merchants blame it on mermaids or somesuch, legends of people riding dolphins and then disappearing under the waves.
Similar legends exist on land but only rural people believe them, mysterious spirits that rise from the underworld to prey on the living. Most people guard their doors when such beings are spotted, none dare follow them back to the underworld.