To go to Koh Sichang you have necessarily to go to Si Racha (buses from Bangkok to Pattaya can drop you at Si Racha) and take the boat at Koh Loi pier. The crossing takes 45 minutes and costs 50 baht. There is a boat every two hours on weekdays and every hour on weekends and holidays.
From Pattaya, you can take the white songthaews 1631 (at Naklua market). It will drop you off in Si Racha at the Clock TOwer 2 km from the pier. The bus to Bangkok will drop you off on Sukhumvit Road.
To leave the island of Koh Si Chang you can only take the boat to Si Racha. There is a boat every two hours on weekdays and every hour on weekends and holidays. The crossing takes 45 minutes and costs 50 baht. The first one leaves Koh Si Chang at 6am.
Once on Koh Loi, the pier in Si Racha, go on Sukhumvit Road at the Robinson Mall to catch buses and minivans to Bangkok or Pattaya, Sattahip and Rayong. Or you can go to the Clock Tower to take the white songthaew to Naklua (North Pattaya)
The train is the cheapest way to travel in Thailand especially in 3rd class. This is also where the contact with the population is the most interesting and incredible...
Taking the plance to travel in Thailand is often not that much more expensive than the bus especially with the numerous specials offers and it is so much faster !