Greetings from Portugal,
If you're a windsurfer like me, visiting Viana is a must!
The hot spot is located at the south end of the river, you should head to Darque.
There you have a camping park, but you can also camp at the beach, there you have direct access to the river side and/or sea, there's a beach shower and a place to rig.
The prevailling wind ihn the summer is from north, and it blows thete for most of the summer time due to the thermal effect of a mountain on the north side of the river. You have two choices for sailing; either blast on the river or exit the river into the sea/beach for waves!
For more info on the spot visit the following links:
www.vianalocals.com - windsurf and surf local shop, with webcam!
www.waira.net - spanish shop with lots of windsurfing pictures taken in Viana
www.wannasurf.com - great site that covers the surf spot's all over the world
I'm sorry to write in english, but my french isn't up to date...