Homopolar generators

Homopolar generators have been around since 1831 when Faraday invented them. Although capable of extremely large direct currents at low voltages, they suffer from poor efficiency because of the brushes usually used to collect the energy off of the rotor, although there are designs that use a liquid metal like mercury instead of the brushes.

Aside from their high inertia and brush-losses there's no reason why they can't be used for electrolysis, although I suspect that a modern PMG (Permanent Magnet Generator) will provide more output for a given input.

The use of hydrogen storage as a load leveling method helps to alleviate some of the utility objections to wind-power, as long as the neighbors don't mind thousands of cubic feet of hydrogen being stored near them.

Both AC/DC converters and voltage converters can economically be built with an efficiency in the high end of the 90% range (>95%). Thus the overall efficiency of the typical used AC generator with rectifier is very likely higher than that of a homopolar machine. I fail to see the possible advantage of a homopolar generator compared to AC generators. It's not very efficient, it's not a natural low speed machine (which makes it not better suited for your mentioned water river/wave power plants) and it has a maintenance problem (brushes).
Last but not least, its high current capability is only due to its low internal impedance - that doesn't necessarily mean that you will end up with a high current in an electrolysis setup.