At this point the paddleboarder is being selfish. It can be quite a dangerous way to exercise. When John went to Spain it was totally legal, and much safer than paddleboarding. It arguably reduced the Uk infection possibilities. I am sure he sees his trip as "normal activity" and needed for his sanity.
if I go out for my local run which even now is totally legal and recommended to keep fit to reduce pressure on the NHS. I too just might need rescuing. The chances are small and a tiny increase in one risk is a price worth paying for a clear improvement in other risks.
The issue is what do you need to do? The main rule to decide that is act within the law, and balance the risks to society in general. In my opinion your paddleboarder did not and John and I did balance the risks.
By the way congratulations on having an interesting job.