[giltleaf]

How much water would this process need to make enough energy for this to have an impact? Presuming we wouldn't use up our fresh water resources for this and that it would come from the ocean, but would this process create potential threats to the marine habitat as the mineral/chemical concentrations of the ocean water shifted?

[AnimalMuppet]

We wouldn't "use up" water at all for this. When you burn the hydrogen (to get the energy from it), it combines with oxygen and forms water.

(Unless you're "burning" the hydrogen in a fusion reactor. But that takes much less hydrogen...)

[foota]

Combustion of hydrogen produces water.