A character on water surface who has no swim speed and has to make swim checks to move is knocked prone by an enemy power. Does this affect him somehow besides the normal effects associated with being prone (combat advantage, -2 to attacks, ...)? In particular, will he begin to fall? And is he permitted to crawl?
Does the situtation change if the character is also unconscious?
In case the rules don't provide an answer, I'm also interested in ways to handle this as a DM.
You're of course welcome to house-rule any special combination of effects if you think it helps with verisimilitude, but the RAWest answer is that it has precisely the same mechanical effect as it does on dry land, it just might benefit from a slightly different description.
Treading water is considered a largely trivial feat in 4e, as is holding your breath. You only fail to tread water if you roll a 5 or under on your athletics check to swim, which isn't possible if you're trained or over level 9 or have 20+ strength. It's 5/10 higher for rough or stormy seas. Falling prone while treading water could be easily imagined as being shoved under the surface, but staying on the top 5 feet (top square) of the body of water. This description would also adequately explain it being more difficult to move (in armor/armed), harder to attack, and easier to dodge projectiles.
Holding your breath (for up to 3 minutes, which is 30 rounds of combat) is also considered trivial, so there's no need to impose any further penalty here, but you could absolutely explain the granted CA as flailing to catch a breath and/or to regain the surface.
It also stands (see what I did there) to reason that recovering from being shoved under water would use your move action.
TL;DR: The prone condition doesn't have any special mechanics for aquatic combat nor do I think it needs them, as the mechanics adequately explain the effects of being "destabilized" in an aquatic situation.
Prone is a condition that can apply to all creatures regardless of circumstance or terrain.
So we're definitely looking at a character going prone in the water being a thing.
How that is ruled is somewhat up to the DM since you have the freedom to model prone however you want if the basic circumstances don't fit (ie lying flat on the ground). All normal conditions of prone are in effect (move action to stand up/go back to normal), combat advantage against melee, +2 defense against ranged etc.
Based on the fact that creatures that are flying or climbing fall when they are knocked or drop prone, I'm going to say that a logical conclusion is that prone creatures who are swimming are going to sink (probably just 1 square as with failing a check by 5+). This isn't in the rules, but it makes sense.
As far as whether or not a creature can crawl, you may attempt any movement method in the water, it requires an athletics check to do so if it isn't a swim speed (or presumably a teleport?). A crawl would have a movement speed of 2 and would end up good for 1 square of movement.
As far as being unconscious under the water, you would simply begin sinking. Check out the endurance skill for guidelines on drowning. Generally you get 3 minutes under water before you need to make checks, so that hopefully enough time that you can healed up and back in the action. Though more details here should definitely be it's own question after you've read that section.
