Ducky Water Spot Remover is the best thing since beer!! I found it last year. My boat at the time I think had every water spot on it since the first owner. I tried paint cleaners and everything else that takes too much effort.
Spray on the Ducky, wait a few minutes and wipe off with a damp terry cloth towel and your boat will look like new!
The best thing then would be to follow up with a nice wax