Bathing suits, suntan lotion and shorts. The tropical climate and laid back attitude of the Keys make this an extremely casual place. There are a few fancy restaurants, such as the Cheeca Lodge, and you may want to check if they have any dress code if you plan to eat there. But, other than that, anything other than shorts is overdressed.
Toiletries and Medical Supplies: Don't forget massive amounts of sunscreen!
Light clothing...shorts, t-tops, t-shirts, sneakers, sandals, bathing suit
Pack light...Less is better.
Its always a great idea to have a light sweater/jacket in case a cool breeze decides to show up, but thats not often.
Always be Prepared.
Toiletries and Medical Supplies: I found that a simple first-aid kit is always handy no matter where or when you travel. Band-aids, extra bottle of water to rinse off any possible cuts...etc...,aspirin, sunblock (this is good for summer and winter)