If you have chartered a "bareboat" it means you do not have anyone to sail it for you. If you don't have the minimum qualification yourself you will need to hire a skipper and ever crew. Check with you charter company who may be able to organise some salty sea dogs for you.
If you have a power boat then you need to know you will normally be charged a fuel surcharge. In the current climate this will be a considerable amount even for day charter.
The effects of the sun are up to three times as powerful at sea as on land. Make sure you take a hat and some sun block. The ladies might be interested in the fact you can tan three times the speed at sea.
Snorkel and goggles are a must if you are going to moor up and take a dip in the sea. They're great for spotting jelly fish before it's too late [a problem in the Med] or sharks [in other seas]
The safety warning are usually recited at the start of the trip and they are worth listening to. Children must wear life jackets at all times. Adults are normally only required to wear them if they can't swim or in bad weather.
If you are lunching ashore it's often a good idea to invite the skipper along. he might have some great sea stories to share with you. It's normal practice to tip the skipper at the end of the trip.
That's all you need to know for now have fun and if it all goes well you should have a smashing day!