Water is both a crucial resource and an environment feature requiring creatures to swim.
Your character is assumed to drink enough water to survive regularly while resting, but you'll need to do so more frequently in order to stave off exhaustion in areas with extreme heat. Drinking 8 ounces of water reduces exhaustion by one level if it's caused by heat.
Moving across the surface of calm water doesn't require any checks. Still water less than 6 inches deep is treated as normal terrain. Water between 6 inches and 4 feet deep can be waded through at half speed.
You can swim on or under water that is at least 3 feet deep. If there is a strong current or some other hazard, you must make a Swim (Athletics) check to fight against it. While swimming underwater, your Endurance save determines how long you can hold your breath.