If Ocean's Hungry Grasp loses all Presence from the board, the joined land remains in play (and retains the printed Ocean area as part of its topology, so makes adjacent lands Coastal), but "Ocean In Play" no longer applies to it, so Invaders/Dahan aren't Drowned after moving there and it's no longer a Wetland for purposes of Powers/Blight (unless it's a Wetland for other reasons).