They are using electrolysis (electrolyzer) on sea water so there is a lot of stuff in there besides pure H2O including dissolved carbon (bicarbonates). This article has some interesting info. “the concentration of carbon dioxide in seawater is more than 100 times greater than it is in air.”

How to pull carbon dioxide out of seawater

MIT researchers may have found the key to a truly efficient and inexpensive mechanism for removing carbon dioxide from seawater. The method could be far more efficient than existing systems for removi...

In the other hand they are discharging water (without the carbon) back into the ocean so I’m not sure what percentage is actually converted into hydrogen. It can’t be as efficient as a plant that is only designed to make hydrogen.

The Equatic Difference | Equatic