Here are some of my suggestions along with some quotes from my readers.
All photos by/of Muffy Aldrich.
Know before:
When and where to meet.
The name of the boat.
Is the boat moored or docked?
How to get through any security?
Where to park?
Who will bring the food.
How many people are with you. Do not show up with more people than were invited.
Wear:
No matter the weather predicted, no matter what the weather on land, prepare for:
Cold, windy rain and
Blazing, hot, bright sun.
“The most common error I encounter is someone not bringing a layer that is warm enough once out on the water, so I stress this, and keep a couple of extra jackets onboard. I think people underestimate how chilly it can get once under sail.”
“One of the few 'passwords of the day' that I remember from camp in Maine is this: ‘Wool keeps you warm when you're wet.”
“I tend to wear quick-dry when racing. Love cotton, but is cold and clammy when wet.”
“Not much is so frowned upon as a guest with too much gear. That being said, plan on the temperature on the water being 20 degrees colder than that on land. Plan clothes with that in mind and then add one more fleece on top! ... Also, chances are that your host has foul weather gear you can borrow if need-be. Ask. Do not feel you need to go out and spend hundreds of dollars. I do support the two pairs of shoes option, but the 'street' pair is to stay on the dock or in the dinghy.”

“Bare feet are not good on boats with a lot of deck hardware as these trip hazards can cause seriously sore stubbed toes. Deck shoes are best.”
“For shoes, look at boat shoes or sneakers. Shoes with black soles are frowned upon as they can mark the deck. Don't wear heels or wedges.”