[RPG] What happens when a swimming creature falls prone

dnd-5emovementprone

My paladin has summoned a giant sea horse as his steed. The giant sea horse has the Charge feature, which allows it to knock creatures prone when it moves 20 feet or more.

Being that the sea horse is primarily a water creature, most of it's charge targets are going to be swimming. So what happens to a creature that is knocked prone while swimming? Does such a condition exist? If nothing happens, it sort of begs the question why they'd include the prone provision at all, since Charge isn't a predefined attribute and is instead detailed in full under the sea horse's stat block.

Best Answer

There are no specific rules for what happens if knocked prone while swimming, therefore, from a purely mechanical standpoint, we must assume that the general rule applies.

From PHB 292...

  • A prone creature's only movement option is to crawl, unless it stands up and thereby ends the condition. (Note: Crawling costs 2 feet of movement per foot moved. Standing up costs half your speed. See PHB 191)

  • The creature has disadvantage on attack rolls

  • An attack roll against the creature has advantage if the attacker is within 5 feet of the creature. Otherwise, the attack roll has disadvantage

So, how to explain this in terms of being underwater?

This is what I have done in my games with two different parties, and it has worked without complaint from my players.

So, what would be roughly equivalent to 'Prone' while in the water? Being knocked off balance, or knocked into something like a tumble. (In this case, you did just get run over by a huge fish...if you get rammed with force that would normally bowl you over while swimming, you tumble instead)

You will have a harder time moving the way you want to unless you put forth some effort to stabilize yourself.

So, you can 'Crawl' through the water in the sense that you can still make some headway while tumbling, but not nearly as fast as you could normally swim. If you expend half your movement, you stabilize yourself ("Stand Up"), and go back to normal swimming.

You have Disadvantage on attack rolls because, again, you're kind of flailing about in the water.

People close to you have Advantage to hit you because you can't really defend yourself like this...but people at range are going to have Disadvantage to hit you because you're tumbling/flailing around, not presenting a consistent profile.

This doesn't make perfect sense from a physics based standpoint (fluid resistance would arrest your tumble, given time...and you'd drift position as you tumbled), but D&D is not a physics simulator.