It's been stated before, but devil fruit users who are submerged in water lose their strength. They can't move or use their powers. The sea doesn't disable their powers, they just can't move or control them.
Eric Miller
2023-10-25 22:10:57 +0000 UTC
This is fan made but it should give you a general idea. I think the Red Line and Grand Line are supposed to make an X, so that North Blue is actually north. You cannot enter the Grand Line normally and it will be explained later. https://www.reddit.com/r/OnePiece/comments/rdsl0w/one_piece_complete_world_map/