1582
Sea Level
(imgs.xkcd.com)
Alt text: They're up there with coral islands, lightning, and caterpillars turning into butterflies.
What he's saying is there is always a bulge of water pointing at the moon (and another pointing away from it) the earth spins so the land sees the bulge coming and retreating as our bit of coast passes under the moon
It's a bad description because the water also spins with the rest of the world, so though there always is a bulge that is stationary relative to the moon, there is always different water being part of it.