Drowning and Suffocation

Suffocating

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 or is choking, it can survive for a number of rounds equal to its Constitution modifier (minimum of 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.
-PHB pg. 183

Cool, but what if you want to drown or choke someone out? Letting something survive for 3 minutes after being forced underwater without a real chance to take a nice deep breath is boring. It ruins creatures that like to drown people. So let's make some slight adjustments (really more of a clarification than an adjustment) for being forced out of breath. If a creature is forced underwater or into a situation where it can't breathe (i.e. a water weird dragging someone to their doom), you skip to paragraph 2. Donezo.