New boss ability. Sea Walker, perhaps gotten from " a great serpent" boss.
New boots, heavy Iron Boots.
What these two items do is—one gives you the ability to breath underwater, the other slows you down tremendously but makes you sink below the water. (You move faster in the water with the boots on than out of it.) The use for these should be obvious enough. Building around water becoming easier, clearing out rocks from rivers would also become much easier. Like most things in Valheim progression tends to make certain things easier and more efficient, while also providing a new challenge.
This idea would probably require new mechanics. Like going to far down would crush you and kill you or maybe this opens up to a completely new area, being the bottom of the sea. A dangerous biome, that forces a time constraint on you; and opens up Valheim to new possibilities. I could imagine a new player discovering the next biome is under the ocean. It'd be really cool in my opinion!
This was just an idea that stemmed from the problem of rocks being in the way of my ships, and the annoying nature of building/excavating in the water. What do you guys think of the idea?