How do I get through the underwater city when only one character is equiped with a breathing helmet. I have tried casting the vine breathing spell, but this only prolongs my eventual death.

Thanks in advance for your responses.

One of the great (and long....) challenges of the Lost Sea is killing enough crabs and taking their shells back to Brimloch Roon to be made into helmets and breathers. The shipmaker takes three at a time and will transform them into the necessary items.

Several strategies have developed to find crabs:
1) sail about and scout then jump overboard to kill them
2) swim along the shorelines, pop under the surface, and kill them
3) temporarily leave some of your party in Brimloch, outfit as many characters as you can, and then dive on Collasium (there are crabs inside that can be killed for more shells) and there are a few areas inside with air pockets that may help

Good luck!