“as the human population increases, shouldn’t that also contribute to falling sea levels?”
Without question. But I think the amount of water that humans retain, percentage-wise, compared to plant life, above ground and under water over the entire surface of the earth is rather insignificant. We’ll probably kill ourselves off before that becomes a problem. (Just kidding, I hope.)
People not only eat plants, but also animals that eat plants. So the human contribution to falling sea levels would probably be interesting.