We recently returned from Patong and whilst there did a high speed boat trip. Lasted about 7 hours, included lunch, and went to 4 islands for sightseeing, snorkelling and swimming. You don't have to swim or snorkel if you chose not too, you can just sit on the beach and relax. The boats carry about 20 people comfortably and you are under cover. Prices of trips are very negotiable and can be booked at your hotel, but we found it cheaper booking through a tour seller on the street - and there are many of these sellers to barter with.