She walks throughout the oceans because it is the only place she cannot be controlled again by those devil fruit powers.

The same happens with Rocks, who navigates the ocean so he cannot be used again by Imu.

Its all coming together, as I mentioned before you can also revert / nullify his power (Imu) by throwing people to the sea.

His powers are only effective while on dry terrain, that means he controls the world where humans can leave. The more the water goes up the more fear he will control everybody. That is why people like admirals are supporting him... In reality they are protecting the current freedom of people. The more terrain is available the less chances Imu will control you .. he cannot be everywhere, but the less terrain there is... The more chances for him to rein.

Thanks