Even if it is raining, the wind is howling, and it is 55 degrees, chances are we will be going out. Life jackets are provided and must be worn. Most of the instructors do not wear a tshirt underneath, and you are free to do the same. Cotton, while it is fine if you would like to wear it, provides no insulation when it is wet, and takes a long time to dry. If gets chilly out at all while you are on the water, you would be better without a shirt on, than with a cotton one.
Suntan lotion is a good idea, even when only going out in the sun for a few hours.
There is some risk in wearing sandals. You can injure your feet on the docks, as the docks float and are not rigidly linked to one another, and occasionally the plancks become warped, making the dock surface uneven. In the boat also, it is quite easy to stub your toe. Although, when only coming sailing for a short while, and in many cases being not so graceful in the boat to start with, shoes are a better choice . Leave wallets, jewelry, and clasp watches in your car .