Swimming

After each hour of swimming, a character must succeed on a DC 10 Constitution saving throw or gain one level of exhaustion.

A creature that has a swimming speed (including a character with a ring of swimming or similar magic) can swim all day without penalty and uses the normal forced march rules in the Player's Handbook.

Swimming through deep water is similar to traveling at high altitudes, because of the water's pressure and cold temperature. For a creature without a swimming speed, each hour spent swimming at a depth greater than 100 feet counts as 2 hours for the purpose of determining exhaustion. Swimming for an hour at a depth greater than 200 feet counts as 4 hours.