Twice now I have knocked out a draconid over water, crashing it down into the water where it could not get back up. Awesome, considering the second time was a level 32 to my level 19 Geralt. Crossbows have their uses!

However, now that it is in the water, what the hell do I do? it just floats there making pathetic noises, but doesn't seem to want to die. Do they ever? Will it ever become lootable? Or is this some kind of glitch that hasn't been plugged yet?

Yeah, probably a glitch. But if it is alive, my guess is that you won't find it in the exact same spot when you come back.

They will usually crash and become stunned for a bit before getting back up, otherwise they might die. There is however a bug in the game where sometimes after certain monsters die, the game still plays sound effects as if they are alive. Grunting noises and whatnot that they make. I've only encountered it once that I noticed it myself with the ghouls on Fyke Isle near the tower.

that only works when they crash on land. Falling into water is insta kill - or so it seems.

Maybe it is a glitch where the animal is stunned, but can't get up so cycles through stunned - awake - stunned constantly. The problem is, their health bar disappears, they no longer act as an object, and there is no way to interact. I'm going with this is a glitch in the system. Will contact CDPR and see what they have to say about it. Hah! I saved and reloaded and it was no longer there. It was gurding loot, and the loot marker was greyed out, so as far as the game is concerned I killed the thing. But I WANTED TEH LOOTZ!

Ahh well...

When they crash on water they might die if the attack drained their health bar, or they might survive and come back up. I've had both happen and it depends on what level the creature is, what level Geralt is and how powerful the attack was just like any other fight. Same thing happens on land though too. Harpies are weak and pretty much insta-kill, while sirens are a bit more powerful and might take a few shots, ekhidna a bit more than that, but if Geralt is really powerful he can take down any of them in 1-2 attacks depending on what buffs are active etc.

When you kill anything in water, unless it is shoe deep the game does not give loot above the ankles more or less. It does not appear to be a game bug but intentional design. One thing I have noticed though is that rabbits that are killed in the wild either by Geralt or by other animals will show up as lootable on the map but you can not loot them. The golden sparkles appear above their entrails, but they're not interactive. That is definitely a bug though.

As I said, the latest time it happened I was 19, it was 32, and it was definitely not low enough to be insta-killed at the time. However it was still making vague noises in the water, which was deep enough for me to swim up from under it.

Since it was gone and didn't return when I reloaded after the fight I conclude that it was in fact killed.

I do actually think this is a bit of a bug, if only because of the cries it was making in the water. However, no more health bar, so out of the fight / dead to the game, or alive and taking very slow damage, but there was nothing to indicate what was happening.