Yes, the curvature works in both ways, but on the beach, when you're looking in the direction, where water is, you can see it only one way, because there is nothing between you and the horizon in that specific direction. When a boat goes under the horizon you can't see it anyhow, but before it does, it gets so small to your eyes, that you can''t see it, even though it's not under the horizon yet, so when you zoom in with some kind of device, you can still see it. Just hold that zoom for a couple minutes or more (depending on how fast the boat is going) and you'll see, that it starts to disappear