Yes, this would have to be a hypothetical question because in your country and many others, it would be extremely illegal to try such a feat. If the swans should get hurt or hurt someone, you could be tried in both civil as well as criminal courts for such issues as cruelty to animals, interfering with a migratory bird, illegal possession/touching property of the Crown, etc.

But, just for grins and giggles, we'll try to answer this. The swans would have to be harnessed to access the most power which would be at the bottom of the neck area as the birds go through the water. Since the wings sit backward on the water, you could not use the wings to pull. Once you harness a swan, it will thrash or completely stop dead in its tracks in the water. The swan would be totally more focused on removing the harness instead of pulling a raft. So, if the swan could be taught to pull, you would only need approximately 2-3 swans to pull the raft as it is light. However, all swans would pull in a different direction or not at all to release the harness, so it really would not make a difference as no pull would take place. The Regal Swan