Swimming

While swimming, each foot of Movement costs 1 extra foot (2 extra feet in difficult terrain such as rapids or fast currents), unless a creature has a swimming speed listed. At the GM’s option, gaining any distance in rough water might require a successful Strength (Athletics) Skill Check.
Additionally, unless a creature has a swimming speed, for each hour spent swimming it must succeed on an DC 10 Constitution saving throw or gain one level of Exhaustion.

Underwater travel

Unless a creature has the ability to breathe in water, it must hold its breath to avoid suffocating.
Swimming deep underwater imposes harsher tests of endurance: higher DCs on their Constitution saving throws, and additional Bludgeoning Damage when they fail. These tests affect even creatures with a swimming speed, though they have reduced DCs on their saving throws. See the Water Depth Effects chart below for a reference.
While swimming in water colder than its body temperature, if a creature fails its Constitution saving throw, it is dealt additional Cold Damage equal to the Bludgeoning Damage taken.

Water Depth Effects

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Depth	Con Save DC	Creatures with swimming speed DC	Bludgeoning Damage on failed save
100 ft	10	-	1d6
1000 ft	15	5	3d6
1 mile	20	10	6d6
10 miles	25	15	10d6
100 miles +	30	20	15d6

Additional restrictions and penalties apply to underwater combat.

Swimming Speed

A creature that has a swimming speed doesn't need to spend extra movement to swim, makes Constitution saving throws with reduced DCs while traveling underwater, and faces no restrictions or penalties during underwater combat.