Swim

A swim speed allows you to move through water as easily as walking on land. You do not need to make Athletics checks to swim, and you move at your full swim speed through water. You can move in any direction while swimming, including diving and surfacing.

While swimming with a swim speed, you are not considered to be in difficult terrain, and you do not suffer penalties for fighting underwater (though your opponents without a swim speed still do).

A swim speed does not inherently grant the ability to breathe underwater. You still need water breathing from another source to remain submerged indefinitely.

Without a swim speed, swimming requires an Athletics check each round. Movement through water without a swim speed costs double, and you suffer disadvantage on attack rolls and Dexterity-based checks while submerged. Creatures without a swim speed who fail their Athletics check begin to sink.