You get water and souvenirs when you get on the bus. If you're not in a hurry, slowly experiencing Hong Kong Island is a good choice. The bus conditions aren't always certain, so don't schedule your subsequent itinerary too tightly. There are explanations on the bus, so you can learn about the local culture, and they'll even help you take photos. Remember to get your stamp before the bus departs, because it'll be too late to get it after you get off.