Swimming and Drowning Rules for 5e
Since this is now something you'll all need to contend with very soon:
Drowning - your character can hold their breath for 1+Con Mod minutes without any kind of issue. At that point (the conclusion of the final minute), your HP drops to zero and you are unconscious and need to make death saving throws (1d20, 10 or above is a win. 3 wins to be stable. 3 failures and you die. Nat 1 = 2 failures, Nat 20 = instant 1 HP and conscious. Given that you can't be stable underwater like that, you'd have one round to continue doing things before you fall back unconscious.
Swimming - You move at 1/2 your walking speed while swimming rounded down to the nearest 5' chunk. So:
Walk 20 = Swim 10
Walk 25 = Swim 10
Walk 30 = Swim 15
Walk 35 = Swim 15