@Sturgis,
I think it is very unlikely that the oceans will ever be dead and gone.
That, of course, doesn't mean that they cannot be degraded to a point where massive extinctions are a result...including, perhaps, humans.
This also doesn't mean that humans should not do what they can to preserve the oceans, but short of blowing the earth to bits, or finding a way to strip it of its protective magnetic field, humans can't destroy it or all life.
There have been massive cataclysmic events in the past and life on earth still thrives.
I've no problem with people who are vitally concerned about the continuing existence of the human species. I would hate to see us extinguish ourselves but there seems to be a need in some quarters to gild the lilly and make "Climate Change" about preserving Mother Earth and all her children. I would think that preserving humanity would be enough for them.