It's apparently vaguely commercially viable to extract potassium from seawater right now, so it probably won't be a problem in the future.

(I came across several mentions of it in the course of looking at the state of phosphorus extraction; https://en.wikipedia.org/wiki/Brine_mining#Potash talks about mining it from the Dead Sea, but the Ocean concentration is apparently high enough to work)

As far as hydrogen, if we can get energy prices down (solar looks poised to do this), there is plenty of hydrogen available anywhere there is water.