I actually recently learnt that most great sea faring nations tended to be worse swimmers than landlocked nations or nations that didn't use the water to their advantage, not sure why though. Probably has something to do with being more focused on moving the ship and not really needing to swim since being a good sea faring nation usually means you have good boats, or maybe it's just pure chance.
As for the harpoon, eeeeh. I'm not how effective a ranged weapon will be that deep under the ocean with the massive pressure of the water on it. And besides, the artifact doesn't even need to be a weapon. Actually, I just had an idea, what if the artifact literally just keeps the water so you can have relatively well armoured troops lead expeditions deep underwater? That would also mean that any sea creatures that got too close would probably start choking in the bubble of air, lessening the effect massive, deadly sea creatures have on the expeditions. Then again the dwarves might of already made artifacts like this long ago, but hey, atleast we would be the first humans to lead expeditions on the bottom of the ocean floor. As for what the artifact is made out of, I imagine being a dwarven artifact full of magic means it doesn't have the issue of deterioration, at least not until MUCH later on when magic no longer exists, but that's just a given.
Edit: A magical urn or water flask seems highly inefficient, since it might have a limited effect depending on how full it is, an artifact that has a proper area of effect like the one I suggested is much more useful. It would also take forever to have a meaningful amount of people to drink out of it before the expedition starts, since at least 100 people be good for an expedition in my opinion.
Editdit: Also for us to be able to send expeditions into ocean would require for us to either declare war against Albion or Hellsite, preferably Hellsite since we still don't know just how much Albion controls that we can't see.