There's no combat action to move another creature to a difference tile by wrestling.
The closest is the "throw" wrestling command, which makes the creature thrown flying for a frame (or frames, no idea really) so that the creature is stunned as if it hit the ground from a fall.
A creature struck can also be sent flying with the force of the blow, but that's hardly a controlled thing, and no wrestling move can do it.
(I remember the first time I saw that. One of my first forts, and the Bronze Colossus came and I had no idea what we were in for. My population of fifty morons were slaughtered, leaving me a guy resting from a sparing injury in bed tantruming in his room, and the dungeon master sobbing like crazy in an alcove the megabeast hadn't stumbled upon yet while the Colossus hunted down every last kitten. Anyway, the first victim got smashed down the entire length of a hallway inside my fort. One of the most vivid moments I'd had. Even though I was barely able to manipulate the interface, and had a hard time knowing what was going on, I could visualize that poor dwarf flying down the hallway past the absolutely horrified faces of his friends to smash into the wall.)
But the correct information has been stated. A carp would flee from a dwarf except for the water not offering them a lot of places to go. But as soon as they are on adjacent tiles (height doesn't count, yeah, you can actually attack any flying thing as long as you are horizontally adjacent regardless of how high it is) they attack the dwarf. If the dwarf dodges, there's a random chance they'll dodge into any adjacent tile, which puts them in the water.
Once in the water, a dwarf with no swimming skills has its speed reduced significantly, not to mention the drowning problem. So the carp with the bite is attacking many times for each time the dwarf acts. It's still a fight that the dwarf can win as long as he's punching of course. If the dwarf is "wrestling" the carp, with no limbs to break, no throat to strangle (eye gouging is a remote possibility, though not very likely) he's just wasting the few attacks he has.
Now, since there's no combat reports in fortress mode, it looks like what people assumed was happening. Dwarf wanders near a water source, and suddenly he's in the water dying. The damn carp must be dragging him in! How did that happen? Urist McFishfood was all the way up on the bank! Carp are dangerous!
But it was really a combination of circumstances. The fact that any creature can attack any other regardless of the height difference. The dodge (normally a good thing now turned against you) that lands you the water. The fact that any murky pool that's full or river is always always always deep enough to drown an unskilled swimmer. (I'm thinking of in our ordinary reality how strange it would be to find a pond that's only 8 feet deep or deeper, and never shallow, with banks that are too steep and just cannot be climbed out of. Now imagine that every single pond ever is like that.) The fact that any murky pool or river cannot be escaped from (without a ramp) by an unskilled swimmer. The fact that an unskilled swimmer's speed is crippled while in the water drowning. The fact that an unarmed fighter will often wrestle his opponent instead of just punching it, in this case a very useless combat action.
I talk too much.