Yes, thermodynamic relationships can be used to determine the maximum amount of electrical energy production possible in the limit of infinitesimally small frictional losses of energy and minimal possible electrical dispersive losses in the generator that transduces mechanical energy to electrical energy. Even under ideal conditions and using the best materials, there will be some dispersive dissipation losses; so that not all of the loss of gravitational potential energy can be captured and converted to electrical potential energy.

Thank you, is the possible energy capture a feasible technological proposal?

I imagine with a barge going down a river also, there would be a constant supply of hydraulic energy.

Yes, this is not only feasible but is being done--one example is some types of hybrid gas/electric cars, which convert some of the loss of kinetic energy (during braking) into electrical potential energy (recharging a large vehicle battery) using fairly conventional generator technology. That braking energy is otherwise 100% lost to heat; heating up you brake pads/cylinders etc.

There are some pilot generators that have been built on seafloor to capture tidal current energy; however many practical problems with installation and maintenance of such generators; generally not economically feasible. I think similar pilot generators have been installed on riverbeds in some places--problems with some back-up of upstream river water (higher river elevation stage) caused by these generators, damage/destruction of installations by floods, as well as overall economic feasibility.