Heres the comprehensive list for England and Wales:
I should expand. Lake Windermere, Coniston water, Ulswater and Derwent Water all in the lake district are the only lakes left in the country where you can use your power boat.
However on all but Lake Windermere there are 10mph speedlimits and as of May 2005 there will be a 10mph speed limit on Windermere too.
There are a few other ski clubs with their own lake around the country but not many will let you put your own boat on.