Fun with Water (Combat rules)

Attack Rolls: All attack rolls are made at disadvantage as a result of having to swim as well as the resistance and other difficulties that the water imposes.
Range: All range based abilities, attacks etc. that has a range is effectively halved. This applies to both long and short range if possible.
Spellcasting: Spells that requires a somatic or verbal component has a 1/4 chance of failure when cast in water. Roll a 1d4 when you are casting the spell, on a 1 the spell fails, but the spell slot isn't wasted.
Movement (Official): for every one foot you swim you need to spend one extra foot of movement. Thus, in most cases your movement speed is effectively halved.
Heavy Load: If your character has total equipment weight of 60 lbs or more it has to spend its action to succeed a DC 12 Athletics (Str) ability check or sink 10 feet.
Drowning (Official): A creature can hold its breath for a number of minutes equal to 1 + its Constitution modifier (minimum of 30 seconds).
When a creature runs out of breath, it can survive for a number of rounds equal to its Constitution modifier (minimum 1 round). At the start of its next turn, it drops to 0 hit points and is dying, and it can’t regain hit points or be stabilized until it can breathe again. For example, a creature with a Constitution of 14 can hold its breath for 3 minutes. If it starts suffocating, it has 2 rounds to reach air before it drops to 0 hit points.