[Radek]

You do not need the undewater breathers - if you have Athletics high (preferably 12), the Breath of Air spell and the Restore Health spell. Also, the Irong Lung trait is extra helpful in Collasium (Thief guild level 5 trait but only for some thief classes. Barbarians can buy the trait. Perhaps, bards can buy it, too. Assassins cannot buy the trait. Another source of the trait is the Ring of Neptune, which you find in Collasium.) I am always doing Collasium without the breather.
If you want the breathers then you need 3 shells for each breather. Take the "quest" in the Docks of Brimloch Roon and sell the shells there. Every time you bring 3 shells you get (can buy? I am not sure now.) one underwater set, i.e., a helm and the breather itself. The breather looks like the shell.
If you plan to go though Collasium without the breathers then test the idea first: swim underwater around in the sea and see whether you are able to restore your air supply efficiently. This might be a problem for big parties. I am always going solo so that I have to heal only one character. Therefore, I am always enough efficient even without the Iron Lung trait.