A change of clothes in case you are cold after practice, or in case we flip (huli). There are showers available, so bring a towel and gear.

What else to bring?

Water Bottle - You will need fluids and calories for the longer workouts.  A sports drink is highly recommended.
Food - You may wish to consider bringing some food for after the workout. 
Hat & Sunscreen - Protection from the elements will make your enjoyment last longer and keep you healthier and better suited for race day.
Shorts - Board shorts or a neoprene short is recommended.  You will get wet. 
Shirts - A technical fabric that will keep you warm even when wet is what you should consider - Capilene® from Patagonia, for example.  Many of the women wear bikini tops under their tops, too.