Try setting your monster-spawn rate at frequent and swimming around the Brimloch Roon Lagoon for a while. I also encountered a number of crabs in the water in front of the Castle area (I want to call it Skull Castle but I know that is from He-Man & the Masters of The Universe LOL..keep in mind I finished my game of W&W in February.
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You can also pickpocket shells off of crabs before you off them sometimes.
If it is not too late for you to take care of other quests/areas, do those and cruise the lagoon a couple of times ever time you come back to town.
I would suppose you could take fewer characters than your whole party into the underwater city but then they won't get the EP's needed to level up if they stay behind sitting around the tavern getting sloshed!
Try the monster-spawn setting at frequent and see how that does for you before ditching your fellow party members.