I'm lost as to how you end up in the middle of the ocean in a boat with a ton of iron in your inventory with no choice but to throw it overboard. You can make multiple trips. Boats have cargo holds. There are stamina potions. Lots of options.

Don't raise the ground under the iron first, raise it around it like a bowl then raise the center so it doesn't slide away.
Also if you are close enough to something you can put a workbench on, you can build a reverse ramp with roof pieces and walk/swim under it to force yourself downwards like for underwater building.