Way too cold in May....?
We start boarding first of April.
The water will be about 50degrees fahrenheit....but thats as good as it gets until summer when it might warm up t 60 degrees or maybe 70 in a very warm year!

I don't know of any places where you can hire a boat for boarding in the U.K., but you can hire tows at the Watermark Club on a lake in the Cotswolds, just google watermark club in cotswold water park and youll find info.