I've come to realize that I need more underwater breathing helmets. How or where can I find more? The ship builder will only sell one.

Be prepared to spend a lot of time searching for and killing crabs. They drop the conch shells you need. Take three shells back to the ship builder and he'll make breathers and helmets. To find crabs you might try:
1) Using a person with the Scout skill to note the presence of crabs while the party is sailing around the Enchanted Sea and then jumping overboard to fight them.
2) Swim along the shoreline popping under the surface to search them out and then attack any you see.

Xignal

Welcome to the board, Edward!!

Hope your search for crab shells will not be too long. Never took me that much time but some players have reported having great difficulty collecting enough shells in a decent amount of time.

Keep us posted!!

Wyv