There’s the obvious: a hat, sunglasses, sunscreen, water bottle, soft-soled shoes, a rain jacket if you like and your personal life jacket if you want. Of course, check the the weather and dress for it.
And the not so obvious: a six-pack of beer, an armful of Gatorades or some snacks, especially once you become a regular. It’s a small contribution compared with that of the boat owner, who bears the costs of club dues, fleet dues, boat and sail maintenance, and dry storage.