When using /fill command to replace water with ex. air it will either miss some blocks or water will regenerate. It is mostly hapends in oceans. The problem is there because when command is executing water regenerates due to command filling every 2nd block. Recommended solution is when using /fill in situation like this to disable block updates for that area, or alternatively implement a command that disables block updates.

P.S. Device: Galaxy J7 Prime G610F