Swans mate on the water. So do geese and ducks. Swan mating starts with a courtship dance. One bird dips its head under the water, then the other does. For some time they carry this on, gradually synchronising their actions. Eventually they actually mate, which involves the pen (female) briefly becoming fully submerged, or possibly the head remaining just above the water line. As the cob (male) dismounts, there’s a little more mutual head dipping to reinforce the pair bond.
I went to West Park recently and, as I got near to the lake, a pair of swans seemed to be at the synchronised head immersion stage of foreplay. There were several other swans not far away, though not that close to the amorous pair. I turned my back as I got my camera out. When I turned again to see the lake, the cob had indeed mounted the pen.
But two other cobs were now next to the pair: they must have swum quite fast while I wasn’t looking to have got so close so quickly. They looked aggressive, with the original cob dividing its attention between the almost submerged pen and returning the aggression of the intruding cobs. The pen, which had previously seemed to be a willing participant in the process, now appeared to be resisting.
Indeed, after a few seconds, she disengaged herself, returned to the surface. Very shortly afterwards she started to swim off at speed, ahead of the cob which hissed at the intruders then swam after her. They disappeared from my view, hidden by the island.