Before swans mate, they engage in a courtship dance, even if they are a long-established pair. For quite some time, they keep interacting, responding to one another. They alternately dip their heads under the water, then raise them beaks dripping. They alternate in upending, so only their tail ends are above the surface. These movements may be performed synchronously rather than alternately. They may nuzzle each others feathers, or dance by waving their flexible necks. Finally they mate, the female almost submerged, before another brief dance.
For lots of the ritual, it isn’t necessarily clear (at least to me) whether their bobbing under the surface is anything more than a search for food. A pair of swans on the canal by Newbridge were, I thought, getting ready to mate. If I’d been closer, I’d have tried to get a brief clip of video to show the way their actions seemed to be directed at their partner.