The drakes seem to be pecking a lot at the female ducks' necks, and occasionally the drakes peck at each others' necks or flap their wings at each other, but more so that the drakes are pursuing the female ducks. I've been observing this behavior for a couple weeks. Whenever the flock of 4 first goes into the water, a drake will pursue one of the females, peck at her neck, then the other drake will come peck at that same female, then a skirmish, then the drakes will turn this attention to the other female in a similar sequence. Generally it has been more or less playful and short-lived. Everybody seems to calm down after 5 minutes or so of this rough-housing and then they all swim happily and peacefully together for hours.
During winter we've been keeping them all in a fenced garden to fertilize it and they don't have any access to a swimming area there, but we let them out on warm days to go swim. Today I wanted to move them out of their winter garden pen and into a springtime pen which includes a seasonal creek as a small swimming area.
Today when we herded the 4 of them into this new pen, they all immediately gravitated to the water area. We closed the gate and just observed them for a while. Unlike previous observations, the rough-housing in the water today just seemed to be escalating and didn't stop. The boys weren't calming down and being in a fenced enclosure the girls couldn't really get away.
There wasn't one drake who seemed more aggressive than the other, so it was hard to decide which one to remove. We let the "alpha" drake ("alpha" designation based on general observations of flock behavior since duckling days) stay with the girls, and I returned the other drake to the winter garden pen, which I suppose is now "drake jail" as you say. After this separation, the group of 3 all seemed to calm down, and the lonely drake was complaining but otherwise calm.
Maybe tomorrow I'll try keeping both drakes together in jail to see how that flows.