To make hydrochloric acid you need a supply of chlorine. You can get that from the salt in sea, but you need electrical energy ( and lots of it) to extract the chlorine. You would also end up with a huge pile of sodium hydroxide as a by product. Sadly iron is never found in its native state and usually found as an oxide. Again you need shedloads of coke, lime etc to reduce this oxide into a metal. Steel plants are not exactly green ! Then, after you have mixed iron with acid to produce your hydrogen, you will have huge piles of iron chloride for which you need even more energy to separate again ! An easier way is to use wind/solar/tidal/hydro power to use water and separate the oxygen and hydrogen through electrolysis. The cycle can be repeated endlessly without piles of waste product. a cheaper and more “dirty” method of extracting hydrogen is stripping the carbon from methane leaving hydrogen to be used with a pile of carbon left behind (coal for all intents) Here we are reliant of a supply of methane.