California uses 1/26 millionth's of the ocean's volume per year (50bn m^3 out of 1.3 bn km^3). So a drop is quite right! According to , it takes 87 mi^3 (~320km^3) to raise the ocean level by 1mm. You'd need several Cali's worth of water use just to negate to that 1mm rise. And that's assuming you destroy the water after use, which basically does not happen.
If you _could_ destroy the water (freeze it all or fuse it all or something) it'd still take a million years to use it up.
But I'd take a wild guess that by the time we get to that level of terraforming, we can build solar reflectors and just prevent the warming in the first place. Desal'ing all that water is going to require massive amounts of power (big fusion plants) plus all sorts of machinery. Tossing a bunch of mirrors into space seems easier to my engineering-ignorant mind.
Couldn't solar or nuclear power allow for desalination pretty easily in countries like these?
Is this a serious question?