What not to wear
Cotton t-shirts
Jeans
Big shoes

Things to have handy in your boat
(optional for weekly paddling, but necessary for day trips)
Drink
High Energy food - chocolate, banana, cereal bars
Small First Aid kit (trip leaders usually have a big one)*
Personal medication - inhalers etc
Dry bag with some extra kit in (jumper + hat)
Torch*
Survival Bag*
Pen knife/River knife*

*not essential but very useful

Don't Forget
Enough dry towels to last the trip
As much kit as you can so that some might be dry for day 2
Extra bits you may prefer to have like nose clips and earplugs
Some very, very, very warm things for post paddling
Shoes that you don't mind getting muddy
Pants

Longer version (quite amusing)