Santa Cruz weather in May will likely be warmish (60s to 80s) and not rainy (if it does rain the locals will freak out). If you're used to east coast weather you'll be surprised by how much it cools off after sunset (Bring a jacket!).
Things you're going to want to pack:
- Camera, of course!
- Enough layers to be comfortable from 50s to 80s
- If you're interested in it, shoes good for walking/hiking (Chaminade also has trails)
- Your favorite pair of sunglasses, the ceremony and cocktail hour are both outside (unless it rains, which is unlikely).
- Suncreen- its always a good idea!
- A Swanky wedding outfit, with some sort of jacket for the evening
- Whatever it is that you'll need for all of the other fun things you're going to do while in CA! (You'll want slightly warmer clothing if you're heading up to San Francisco).