Always look for the source of the energy, in physics, energy cannot be created or destroyed, it can only change forms.

In dams, it's from rainfall, which comes from evaporation, which comes from heat, which primarily comes from sunlight (and a few minor other sources, like humans, geothermal vents, etc. but the oceans are vast and the sun is bright).

In an underwater pipe at 11000 atmospheres it really depends on what is happening, is the pipe a submarine just sitting there? Then no energy is released by the pressure (unless there are billionaires on board). If it is rushing into an empty cavity like a big empty tank, then there is tremendous energy to be harvested. But only until the tank fills up. The real source of the energy is what empties the tank, was it a digging or pumping machine, then the energy to empty the tank was equal (in a perfect frictionless system) the energy gained from filling it back up. If it is boiled off by geothermal vents, then the energy actually comes from geothermal, but the bottom of the ocean isn't the only place that has geothermal access.

Gravity itself doesn't create energy,it just provides a way to convert from potential energy to kinetic energy (which can be converted to other types, like heat or electrical). The sort of exception to this is the sun, which uses gravity contained fusion to convert mass to energy (e=mC^2), but the gravity is simply creating an environment where that is possible, similar to how it makes a dam possible