a raiseed gallon of water can only have so much potential energy, however...?

A raised gallon of water can only do work = its gravitational potential energy [ m g h, where m= mass; g = acceleration due to gravity; h = height to which it is raised]

That's it. It can't do any more than that. Work is only done when the point of application of a force moves in the same direction as the force. So movement has to be involved. For a cork floating on water, apart from the initial displacement of the water, there is no movement involved. Once the cork has settled, the water is doing no work at all.

You are confusing between gravitational energy of something and what work you think it is doing. But when it lifts a cork or sheep it is doing it at the cost of its hydrostatic energy, if you convert into steam it can do still larger amount of work and if you can electrolyze it and burn the hydrogen so obtained still larger amount of work can be done. When you raise a gallon of water the energy of the raised water and earth system gains in energy, which you can think as stored in the raised water. If this raised water is allowed to fall down it will get an equivalent amount of kinetic energy which can be utilized to produce electric power. The kind of work you are bringing in here has nothing to with the act of raising the water. that work it will do anywhere.