This achievement can be gained in the level with the submarine.
The plumber's remains are in a little cave just above and to the right of the submarine.
Once you have jumped in to the fish body, you will be able to freely swim around, and will be able to reach the cave.
Just swim to the little cave and either lead the little worms away, and then return to the cave, or quickly swim up to the remains and click
before the worms kill you.