Whoops! I've never intentionally let one of my characters "drown", although that would be justification for using the "Resurrect" spell
. Alternatively, you can leave some of them behind in the Inn in Brimloch Roon...just make sure they're not carrying anything important (like the Tomes of creation!) The Inn has a propensity to "eat" items.
On a side note, I've never had any difficulty equipping all my guys with breathers and helms. Granted, I play with a 4-character party rather than 6-character (6 characters tends to get redundant later on, IMO), but even when I had a 6-character party in my first play-through, I was able to equip everyone with breathers. You have to be patient; patrol the seas between Brimloch Roon and Skull Island; that's where most crabs spawn. Turning Spawn Rate up to "frequent" helps mightily as well. And it definitely helps having a character good in scouting, as they will "alert" the others when a crab is nearby.
-Sazerac