Water availability is a red herring IMO.

Desalination is expensive, but even desalinated water would add very little to the cost of green hydrogen.

A high-end cost estimate for desalinated water is about $1.50 per kiloliter / metric ton. There's about 111 kilograms of hydrogen in a metric ton of water.

Even assuming only 50% of that ends up as saleable hydrogen, you're still talking less than 3c of desalination cost for every kg of hydrogen you produce. The other thing to keep in mind here is that hydrogen production occur only at places and at times where energy is very cheap, so the energy inputs to the desal plant should also be very cheap, pushing down the cost of desalination.

1) The energy cost of the desalination would have to be added to the energy budget of the hydrogen production.

2) The atomic weight ratio of oxygen to hydrogen is something like 16 to 1, even with H2 and O1 you end up with a ratio of 8 to 1, so about 100g hydrogen in weight per kg of water.

1) My point is that the actual market cost of desalinated water includes the energy cost - and probably more expensive energy than a dedicated desal plant for hydrogen production would because both run at times and in places where energy is cheap.

2) My calculation above already took the mass fraction into account, and added a generous fudge factor in case not all of the desalinated water can electrolysed and some ends up being wasted.

2) Completely missed the "there is 111 kg ... " part, my apologies.