Every 10 of your turns, water rises one rank from your back rank. You can't move underwater pieces or to underwater squares.
Rising Wateris one of Nerf Chess's secret handicaps. In Nerf mode you choose one of two nerfs before the first move, in secret, and carry it for the whole game.
The board quietly enforces it: any move it forbids is simply never offered, so you cannot break it by accident. Your opponent never sees which nerf you took until the game ends, so every move you make is also a clue they are trying to read.
“The tide is rising.”