What happens when a swimming creature falls prone?

Question

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.

Answer 1

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.

Answer 2

The same thing as any other prone creature

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Official ruling

Jeremy Crawford has confirmed the answer that follows as a correct interpretation:

If you're knocked prone underwater, you're subjected to the effects of the prone condition as normal. One way to visualize it is that you're floundering.

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There are no conditions/rules/restriction in the game about when a creature can be prone. So when a game effect says that a creature is knocked prone, they follow the same rules as any other prone creature.

The effects of being prone are:

• A prone creature's only movement option is to crawl, unless it stands up and thereby ends the condition.
• 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.

To flesh out our understanding of being prone underwater though we should look at a few helpful things:

Prone doesn't necessarily mean flat on the ground

I think it is important to note the prone is a condition in the game, but not necessarily always a physical description of the physical state of the afflicted creature. The primary purpose of the condition is to describe the mechanical effects on the character. Thus prone could mean different things in different circumstances.

I cannot find anything explicitly addressing this, but we can learn from an example that is addressed: being prone while flying.

Prone condition also exists for flying creatures

Firstly, a flying creature can be knocked prone:

If a flying creature is knocked prone... (PHB)

That makes as little or even less sense than a swimming creature being knocked prone in the common sense of the word. Yet it is explicitly possible and would entail the same mechanical effects as any other creature being prone (with the additional downside of also now falling but that is specific to flying creatures).

Secondly, according to an optional rule:

...it can halt the fall on its turn by spending half its flying speed to counter the prone condition (as if it were standing up in midair) (XGtE) (emphasis mine)

The fact that it says "as if" it were standing up tells me that the game is acknowledging that prone is not literally describing the physical state of lying flat (which as we talked about above, would be a ridiculous if not impossible state for a flying/falling creature to be in). Thus this is a mechanical effect more than a descriptor of the physical state of the character.

Swimming creatures should apply prone the same as any other creature

This applies equally well to the thought of swimming creatures. As such I see no reason why prone should be any different for swimming creatures as it is for flying creatures or creatures on the ground, even if it makes little to no sense physically.

Describing underwater prone in-fiction

If I were DMing a situation where a swimming creature was knocked prone I might describe it to the character as the creature getting flipped over to an inconvenient axis of their body and having to struggle to right themselves to function normally.

Answer 3

Jeremy Crawford has clarified his clarification:

Being underwater doesn't stop you from being knocked over. Hovering also doesn't stop you from being knocked over, but hovering does stop you from falling when you're knocked over in the air.

and further clarified the clarification:

You can almost always be knocked prone. About the only time it's physically impossible for you to be knocked prone is when you're affixed to something that keeps you upright.

Thus, the answer below is wrong.

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There are cases where a creature cannot be knocked prone

Jeremy Crawford has posted a clarification:

Q: If a creature is swimming, hovering or is otherwise unable to fall (e.g. chained to a wall), does knocking them prone actually inflict the prone condition?

A: If it's physically impossible for you to be prone, you're not subject to the prone condition.

Jeremy Crawford implies that using the general common definition of prone is what is intended here: "lying flat, especially face downward".

A very compelling argument could be made that, when swimming, buoyancy and other factors would prevent one from physically being prone. But I think that, in the end, the DM would have to decide based on the specifics of the situation which is the correct interpretation at the time.