I am not refering to the curvature of the Earth causing the curvature of the water around its surface.
I'm talking about the fact that a single ocean can have hundreds, if not thousands, of "mountains" and "valleys". And no, I'm not talking about waves, or the tides due to the gravitational pull of the moon.
However, it does, indeed, have something to do with gravity.
The Earth's gravitational field is not completely even. Granted, the differences are so small that we can't tell any difference between one place and another on our own. But there are tools which can.
Take a large land mass, like a mountain. If you stand next to it, you don't notice that the mountain is pulling at you ever so slightly away from Earth's Gravitational Center. But, if you just happen to have a plumb-bob, and hang it there at the edge of the mountain, you would notice that it is not hanging exactly verticle. The little weight is attracted a tiny bit towards the mountain, so gets pulled toward it enough that you can actually measure an angle from vertical.
Well, the sea bed is not just a flat layer of silt. There are underwater mountains and valleys as well as above the water. And where there are mountains, there is a slightly higher gravitational pull. And in the opposite direction, where there are valleys of less mass, there is a smaller gravitational pull.
These gravitational differences are small, but they still have an impact. For instance, the water level above a mountain can be as much as 30 feet higher than the average level. And in some of the bigger valleys, the water level can be as much as 30 feet below the average.
These differences cannot be seen from any ship or from land, as the gradation from sea "mountain" and sea "valley" is so slight. Instead, a satelite in orbit around Earth was used to measure such differences in 1978, called, Seasat.