Could hydrogen liberated from water provide lifting energy which exceeds the energy it took to liberate it from water

I haven't done the calculations, but I doubt that this scheme would generate net energy. As was pointed out, electrolysis uses a lot of energy. However, after the H2 and O2 rises up the water column, you could get some of the energy back with a fuel cell that would convert the hydrogen and oxygen back to water and supply additional electric power, but inefficiencies at each stage would have to be made up from the energy gained by the rising gases in the column of water.

To show that there has to be a net overall loss of energy, consider the following modification to your problem. Imagine a long vertical pipe filled with water. Now instead of electrolysis, let's say you instead use an air pump to inflate a balloon near the bottom of the water filled pipe. This takes work which goes into lifting the column of water up such that the surface of the water rises enough to allow for the volume of the ballon. Once the ballon has risen through the water column the water level will go back to it's former position. That fall in water level is the source of the energy that the rising ballon could generate. So there is no free lunch or perpetual motion machine here - inefficiencies at each stage will insure that there is a net loss of energy.

By the way, I do not know this for a fact, but by this thought experiment, I would predict that electrolysis of water under high pressure would take more energy than under lower pressure. I say this because the electrolysis is effectively inflating a balloon against the pressure of the water - which will thus take more energy at high pressure.

The answers stated so far has neglected to take into account the pressure factor. (@George comment). When you try to do any chemical process, you need to pay the energy to overcome the entropy of the system and not only the potential, this is why we consider the free energy of something and not the potential energy.

If you don't consider that, you can replace water with theoretical chemical, let's call it amazingume, which has similar properties of water except that it is easy to do electrolysis to. You still won't get that desired free energy since you'll have to pay the energy of putting the amazingume elements in the atmospheric pressure.