I went to Tioman in August last year and took the public bus. As others have mentioned, take any bus that goes to the customs, and take a bus to Larkin bus terminal from the Malaysian customs. At Larkin, there are several bus companies that operates buses from Larkin to Mersing. You can buy the bus tickets over the counter (which I did), subject to availability. There are usually at least 1 bus leaving every 1 or 2 hour throughout the day, so if the bus is full, the most you have to wait is around 2 hours. During peak season, you might want to book the tickets in advance to be safe, in case the tickets run out, especially if you have specific timings to meet (chasing the boat timing, etc).