Can anyone walk under water IRL... I mean without needing a wetsuit and pounds of weight added.. so why would you want to in this game?

Water alone, in the North, where Vikings lived is exceedingly dangerous due to the cold, and in this case attempting to swim in any armor, in cold water, and fighting should sap so much stamina you should die anyway.

Perhaps the answer here is, don't fight in the water.. that's my take on this.