I saw two ducks doing it today at Lake Ella. It happened like this:
The smaller one was in a shallow area of water, and the bigger one stepped up on the other's back, like they were a pair of burglars in a bad comedy flick hopping over a security wall at night. The one standing up looked around a while, and then it started kicking / stomping the one underneath. It was pushing the bottom duck's head in the water and holding it down. Then it lowered down and kept pecking at the lower duck's head, dunking it in the water for long periods.
The bottom one remained still, and halfway through, after I hadn't seen it move the whole time, I thought that the bottom duck might have been dead. The top duck could have been picking pieces out of it, trying to eat it, like hungry lobsters are known to do.
Near the end, another duck came swimming quickly toward the pair. Some small turtles also gathered 'round. I thought that they were trying to rescue the lower duck.
(Homer thinks back to the way he learned about sex as a child...
Homer: "Zookeeper! Zookeeper! Those two monkeys are killing each other!"
Zookeeper: (whispering) They're doing it.)
It was all over within three minutes or so, then both ducks waded to shore and spread their wings, elegantly flicking the water off their fowl bodies.