Seems to be some confusion and I thought I'd stir the pot a bit. I prefer these definitions:

kook

a kook is a poser who turns up in the beach car park wearing a top of the range wetsuit and carrying a spanking new surfboard, checks the surf for an hour before actually going in, when it is so obviously pumping, then when he enters the water proceeds to splash around like a drowning rabbit.

kook

Kook is a term, most often used by aggro locals, to describe any surfers that:
- don't live in the ****hole little coastal towns
- don't work construction and/or drive old, beat-up trucks
- pretend like they can surf when, in reality, they suck-ass
- don't follow the rules of the lineup
- show up in the lot with a frappachino, excited about 2-footers

However, the term kook has a different meaning to different types of surfers. It is most likely the case that every surfer, at one point or another, has been called a kook themselves and has called other surfers kooks. You don't have to be an asshole local to enjoy this fun, degredating term!!!
"That guy has a surf-rack on a land-rover, what a kook!"
"Hey Kook, get off my wave!!"

Source: http://www.urbandictionary.com/define.php?term=kook