This could have been fighting by two males either fighting over a female or one male protecting it's mate and habitat. Sometimes, fighting males will get on top of another to show dominance and heads with go under water and stuck between wings. This looks like mating and could be two juveniles trying to mate with each other or a female.

The difference in mating is that the swan's neck would have been held above the water even though it might have been seen underwater. But, the mating swan on top if it were mating would have tried to keep the head above water. If there was a lot of chasing and thrashing with the wings, more than likely a fight. If the swans immediately formed a heart with their necks, facing each other and talking to one another, then it was mating. The mating swans would then separate and begin bathing. If it was fighting, the loser would have gone as far away as possible and try to stay out of sight from the Alpha male. The Regal Swan