Personally, I prefer using the breathers than trying to do Collasium without them. Far less concern about your characters drowning.
The scout skill is, as mentioned, very useful to locate crabs. I just sailed around in my ship until my scout said there were crabs nearby. Then I would jump overboard and find them. You can pickpocket shells from crabs sometimes so that's worth a try.
You don't have to take your whole party with you to Collasium. Many players have gone with only 3 members of their party to avoid having to collect shells for all. I like taking everyone!
Wyv