Used to live in a riverside house in Wroxham, but we moved to Kent too long ago (1987) to know who is good or bad at the moment, both Wroxham and Horning both used to have some good hire companies.
another place to google would be Horning, which has several hire yards for durations from one day onwards.
horning was the home of Tom Percival's boatyard, he and his compatriot Bob Spalding were the top British power boat racers in the 1970s and early 1980s. Tom sadly died in 1984 in an accident at Liege, but his yard is still there, now owned by Southgates (although they no longer do boat hire).