Interesting topic, I think they should be categorised into types.

Undead - They are supernatural so stamina is not really and issue, they are dead, not going to get tired, so swimming they do not need stamina.

Living/breathing types - should have stamina at least for swimming, they already have pseudo stamina in their attack speed, otherwise you would get attacked a lot at start and then they pause. Instead the mobs can be adjusted by attacks per time for difficulty and not need to calculate stamina per se.

Viking - it is a annoying to have weapons pocketed when wading, maybe it needs a slight adjustment rather than allowing drawn weapons while swimming. Maybe can draw a knife when "treading water" rather than swimming. Though you should try swimming while holding a couple kg of weights and see how get on. So I am suggesting adjust the depth at where the weapons get put away.