Underwater Rules

Swimming, PHB p.182:
While swimming, each foot of movement costs 1 extra foot (2 extra feet in difficult terrain), unless a creature has a swimming speed. 
At the DM's option, gaining any distance in rough water might require a successful Strength (Athletics) check.
Simplified 3D Combat:
When using Josh's simple model for 3D combat, we'll make use of 3 'levels' of space. Namely, these are 'the surface', 'submerged', and 'deep'. For simplicity's sake, the distance between things on different levels is their distance apart in 2D space plus an additional 15 feet per level apart. 
A creature with a swim speed can traverse between levels by using 15 feet of movement. A creature without a swim speed may do so by using either all of their movement or 30 feet of movement; whichever is less. 
For example, a halfling rogue may travel from the surface to the submerged level by using all of their 25 feet of movement. A level 3 human monk may do so by using 30 feet of their movement, and will then still have 10 feet of movement left. 
Two of the colour icons found in the character token status options are used to represent what level of 3D space a character, monster, or object is currently on:
- The surface remains as normal, with no colour attached to it. 
- The 'submerged' level is marked by the blue icon. 
- The 'deep' level is marked by the purple icon. 
Underwater Combat, PHB p.198:
While underwater, the following rules apply:
When making a melee weapon attack, a creature that doesn't have a swimming speed (either natural or granted by magic) has disadvantage on attack roles unless the weapon is a dagger, javelin, shortsword, spear, or trident. 
A ranged weapon attack automatically misses a target beyond the weapon's first range increment. Even against a target within this range, the attack roll has disadvantage unless the weapon is a crossbow, a net, or is one of the following thrown weapons: javelin, spear, trident, or dart. 
Creatures and objects that are fully immersed in water have resistance to fire damage. 
Holding Breath and Drowning, PHB p.183:
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 stabilised until it can breathe again. 
For example, a creature with a Constitution of 14 (+2 modifier) 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. 
Note from Josh: In the event a creature takes a big hit while holding their breath, I may call for a constitution saving throw, with them losing some of the time they can still hold their breath for on a failed save - as if some air was knocked out of them. 
Casting Spells with a Verbal Component while Underwater, from this Sage Advice tweet: 
"No rules prohibit using verbal components underwater. However, keep in mind that if you're talking, you are not holding your breath."
i.e. If you are holding your breath, you can cast 1 spell with verbal components while underwater. After this, you immediately begin drowning and follow the rules detailed above. Creatures that can breathe water (naturally or through magical means) are unaffected by this. 
Heavy Armour and Water
If a character wearing heavy armour is incapacitated while in the water, they sink at a rate of 1 level of space (as per the 3D combat rules) per round until they are in the 'deep' level, where they remain.
Category:Mechanics and Rules