I have some doubts over Jake's assertion that the Met Office Inshore waters forecast '...is used most widely by all RYA Instructors....'. As an RYA Instructor, I certainly don't use it. If you lived on the coast, had no access to the internet, and just a radio, or if you were planning to make a coastal passage, then it probably is the best available. But if you have internet access, there are the many more sources which are able to go into much more detail regarding your actual location (rather than the many square miles of sea covered by each of the Inshore waters forecast areas). Jake mentions Some of these.
The weather forecast websites Jake mentions are: