We already know the ocean's topography fairly well, we just haven't visited all of it
We've been to the deepest point of the ocean (that we know of)
3 people have been to Challenger Deep. 24 have been to the Moon. 12 have walked on it.
It's also generally easier to live in space than the very bottom of the ocean because in space, we have pressurized spacecraft, while in the ocean, the pressure of all the water above you will crush anything that stays down there for more than a few hours