Whatcom Paddlers Discussion Group group of avid racers and downwind paddlers based in Bellingham, WA. USA Apply for membership - getting and keeping access is usually straight forward as long as you are legitimately interested in paddling and not a spammer.
Rambo's Locker Ian Newland doesn't seem to be adding to this anymore but there are good past posts. (Is Rambo still transfixed with Ninja Warrior training?)
Marathon Racing
Midwest Performance Paddling canoe and kayak racing (website can be a little hard to view - scroll down to get menus into view against background)
River Miles forum focused on river and marathon racing
Sea Kayak
Sea kayaking forums can be a good source of information on tides, weather, tidal races, good launch sites, etc.
The Paddler's Guide Just about every kind of paddling other than surfskis (these guys haven't really caught on about surfskis yet) but they may have some gear advice that is helpful