Don't know if this is feasible, but following the "theory" that some parts of the Nether are colder than the rest of the dimension (specially Soul Sand Valley and perharps Basalt Deltas), an option to make water to be able to exist only on those biomes (trying to place in the rest like Nether Wastes) would fallback to vanilla behavior, or in case of flowing water, when it reaches the end of an allowed biome its flowing block should stop spreading to the "hotter" one.
Don't know if this is feasible, but following the "theory" that some parts of the Nether are colder than the rest of the dimension (specially Soul Sand Valley and perharps Basalt Deltas), an option to make water to be able to exist only on those biomes (trying to place in the rest like Nether Wastes) would fallback to vanilla behavior, or in case of flowing water, when it reaches the end of an allowed biome its flowing block should stop spreading to the "hotter" one.