Here is what I came up with, based on previous editions, and consistent with the styles of 5th edition rules. • While swimming, each foot of movement costs 1 extra foot, unless a creature has a swimming speed. To swim or stay afloat, you might require a successful Strength (Athletics) check. If you fail by 4 or less, you make no progress. If you fail by 5 or more, you go underwater. Calm water is DC 10, Rough water is DC 15, Stormy water is DC 20. Apply an encumbrance penalty to the Strength (Athletics) check: -1 per 10 lbs for creatures up to Medium size, -1 per 20 lbs for Large, -1 per 40 lbs for Huge, -1 per 80 lbs for Gargantuan creatures. In addition, I slightly modified the rules for drowning to make them seem more "realistic." • Asphyxiation — When a creature runs out of breath, it gains one temporary level of exhaustion per round, then drops to 0 hit points and is dying when it reaches level 6. If revived, the creature recovers from all levels of exhaustion that were gained from asphyxiation.