Originally posted by dr.toast:

If single player, you can always pop the devcommand and enable fly.
That lets you dive right to the ocean floor at will.
Personally I'm okay with it cos its a bit BS that stuff sinks like that.

Also for legit ways, building a small ladder and "diving off" it spamming e to pickup can work depending on how deep.

The image in my head of a viking suddenly flying up and then into the ocean seamless causes me the chuckles. I guess it is a form of the after life so it could make sense.