Using the by-product of the fuel cell reaction is dicey if the anodes and cathodes aren't the expensive platinum-group ones that won't form new substances in the water as the current passes through them and the water. Also, the concentrated brines formed when water is split are no better for the environment than those produced by desalination plants.
There are much better and safer ways to generate drinking water. In the initial stages, there are easier and more direct ways to generate and store energy than hydrogen, which usually needs to be compressed, unless storage is in a giant bladder, in which case the gas is extremely flammable.
Personally, if we're talking about untested technology, I would rather go a more grounded route. Perhaps we could strain plastic refuse from the oceans and incinerate them at high temperatures to generate our energy, such that the only by-products of combustion would be carbon dioxide and water vapour. That exhaust could then be used in a
greenhouse, on-island, to improve growing conditions of non-food plants, potentially accelerating the expansion of the mangrove and salt marsh barriers.
Stacking functions is lovely, but if we get too enthusiastic about it, unintended consequences start piling up.
-CK
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
-Robert A. Heinlein