Firing warning shots legal or not?
Well I''m not an expert on maritime law, but I do know a bit about land criminal law. On the land if you fire a gun at or towards someone you''d better have a darn good reason or you''re gonna be the one the local authorities are going to be spending most of their time looking at. Using a lethal weapon to warn someone about annoying behavior generally is frowned upon by Law Enforcement.
As for the annoying motor boatists you might look to see what law enforcement agency covers the area of water you are mainly in. In my area the local sheriff''s office has a boat patrol that deals with the local river and lake boaters (no sea in this area). I know there are strict rules of boating in this area and they enforce them. If you can get a bow number or other identifying marks of the offending boat and then call the police they can hopefully get a boat over or have a patrol officer contact them at the dock. I know this doesn''t have the immediate gratification of a "water rage" response but you may be able to get the annoying boatist kicked off the local water (it''s happened here) and that will make things more peaceful for you in the long run.
Hope that helps even a little,