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.

That's what I do, if I cannot get to things by jumping off the bow of a boat.
Note: to go to the bottom of the water, use the control key.