I think its a personal thing.
Personally I love Billygoat Productions stuff.
I have "Because" and its a great film, not a documentary, basically mostly surfing with background music so ideal for non-sound watching as no long boring bits.
http://www.bgdsports.com/videocart/m...%7CSurf%20DVDs
For clarity of picture, I think "Step into Liquid" is amongst the best however it has long non-surfing sections and interviews and so probably isn't as good for background video only.
At the moment Billygoat are doing "Because", "Irakita" and "Better Days" as a 3 DVD pack for $59 which is about £30. Not seen the other films but supposed to be good so could be a good option for background video as would give you a choice of 3. Buying from the states is usually OK as BG productions are usually Region Free (best to check 1st). I've bought them from the states before - much cheaper.
Al.