It isn't instantly converted back to water. Even if it were, it doesn't instantly go back into the ocean. This over simplification of "hydrogen means free energy" is dangerous and is reminiscent of the early days of nuclear and fossil fuels. Don't flame me for wanting to consider all the possible negative effects. Neglecting potential downsides is why our planet is in such a terrible state now.

Has anyone considered the effects of increasing the salt concentration of the ocean? And don't tell me "the ocean is big, we couldn't possibly have an effect" because that's what climate deniers have been saying the about the atmosphere for decades. Our fresh water supplies were massive at one point too and look what we did to them. Our water needs are growing and will continue to grow. We need to consider and account for all potential side effects.