Re: And why is it so slow?
"It's simple - ring deserted coastlines with wave power generators to crack the seawater"
Could I take a reasonable guess that you didn't study chemistry at school? Electrolysis of brine does indeed produce hydrogen. It also produces other things. At the cathode you get hydrogen gas, at the anode you get chlorine gas, in the residual brine you get sodium hydroxide. Disposal of the chlorine and sodium hydroxide is problematical. In the not-so-distant past the headwaters of the river Mersey were dead. This was because the cotton bleaching factories used sodium hydroxide to clean the cotton before bleaching and chlorine to bleach the cotton. The waste was piped into the Mersey and its tributaries. This killed most of the life in the river for a considerable distance - from Stockport to the sea. The factories used only a small fraction of the sodium hydroxide and chlorine that would have to be dumped from producing sufficient hydrogen to power cars.