I visited Sentosa Island last year. Its beaches did not seem dirty to me. It was indeed very hot, but I stayed in the hotel during the hottest hours. It was a nice hotel. I made the reservations through RihlaPrime. I came across some of their offer. They typically cut down 50% fro the original price. Anyway, I consider my experience quite relaxing and entertaining. Sentosa also has an adventure park that you might like.