> "Ocean tides have a power of about 300 gigawatts"
Not even sure how to apply that number. Consider that the reduced gravity of the water at the thermocline may by orders of magnitude less than at the surface. Therefore to move cooler or hotter water up and down in the ocean takes a lot less driving energy than one would imagine. For a density differential of 0.1% at the thermocline, 300 gigawatts of tidal power could easily effectively look like 300 terawatts. However, this is not dissipated as it is just moving the heat around.
The ocean is essentially a finely balanced see-saw, most recent research for ENSO applies reduced-gravity ocean models to simulate the seesaw (see e.g. [The El Niño-Southern Oscillation Phenomenon](https://books.google.com/books?id=qWNEO-zkr9IC)). When both ends of a see-saw are weighted similarly, all that matters is the differential force. That's the idea of an effective gravity.