I recall a kooky idea to capture energy using the temperature differential between seawater at the surface vs. much deeper. Can't for the life of me remember how it worked, but I do remember:

it was a vertical device that bobbed at the surface, but extended a long way down to cooler water.

it held a fluid that boiled/bubbled/foamed.

fluids were contained and recycled from bottom to top to bottom...

What I can't remember is the gimmack that permitted the colder part at the bottom to boil. And the warmer part at the top to reliquify. Something's funky about my recollection because it doesn't add up!