If your doing Comp Crew then they'll give you Wellies, Salopets (sp?) (trousers) and waterproof jacket. You just need to put warm kit underneath. Socks, gloves, towel etc. Trapping gear as well in case you get to Southampton or somewhere dry. Put your gear in a bag which can get crushed up, make sure it's waterproof as condensation is a bitch.
Don't worry about the weather not being great, thats half the fun!
Edit: Take some cash for mooring fees, it costs to park each night!