.. I'm at the bottom of the ocean.

The trick here is to think beyond the obvious.

The obvious would be to try to grab one of the swords, knives, sets of scissors, etc.

You don't need anything sharp to get out of the water.

The rope is tied to the idol.

The idol is affixed to the ground.

If cutting the rope didn't work...

Maybe removing the idol from the ground would.

Pick up the idol.