Keep the following in mind when underwater:

Movement

Unless a creature has a swim speed, its movement is halved.
After each hour of swimming, a creature must succeed on a DC 10 Constitution Saving throw or gain 1 level of exhaustion.
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

Attacks

Melee weapon attacks have disadvantage (exceptions: dagger, javelin, shortsword, spear, or trident).
Ranged weapon attacks made outside of the base range automatically miss.
Unarmed strikes have disadvantage.
All creatures fully immersed in water have resistance to fire damage

Holding Your Breath

A creature can hold its breath for a number of minutes equal to 1+ their Constitution modifier (minimum of 30 seconds). Make note of this total number of rounds when you dive underwater!
This number represents your Breath. A Constitution modifier of 2 means you have 1+2=3 minutes, or 30 Breath.
The following things can impact your breath:

  • Every turn in combat reduces your Breath by 1.
  • Dashing reduces your Breath by 1.
  • Taking damage reduces your Breath by 1d4.
  • Taking damage from a critical hit reduces your Breath by 2d6.
  • Casting spells with a Verbal Component reduce your Breath by the spell's level.

Drowning

A creature can last a number of rounds equal to its Constitution modifier (minimum of 1) when they run out of oxygen before they drown.
When a player begins to drown, their hit points fall to 0 at the start of their turn and they cannot receive the benefits of a healing spell or potion until they are able to breathe again.

Vision

  • Clear Water, bright light: 60 ft.
  • Clear Water, dim light: 30 ft.
  • Murky water or no light: 10 ft.