Water

Swimming slows you down. Electrocution hurts far more in water, as do Power Sap blasts and the Ghost Gibber. Stomp can't hit swimmers, and swimmers can't Stomp. Molotovs that land in water don't explode. A drug or poison mixed in with a Water Pump, or by hacking a computer to poison the water, affects everyone swimming in it. On the second floor of each district, it freezes into ice.