Ok. So looks like a condition called 'water tail' which @ruston has mentioned. He actually hates water and has not been swimming but it is not unique to swimming apparently. Over waging can also cause this to happen. The vet also noted him lame on his left rear and sore above his tail when touched.
Anti-inflammatories, rest and time.
At at least nothing serious. A lot of people comment as it is still quite unusual to see a Rottweiler with a tail so I am glad it will be wagging away again in a week or so.