Buses to Si Racha leave from the Eastern Bus Terminal (Ekamai) and Mochit Terminal every 30 minutes. Bus fares are slightly under or slightly over 100 Baht depending on where you start your journey. Also many buses including mini buses heading towards Pattaya will stop in Si Racha. There is even a minibus that runs between Suvarnabhumi Airport and Sri Racha which costs Baht 100 and takes approx 1.25 hours.
When you disembark in Si Racha take a tuk tuk or motorcycle taxi to Koh Loi Pier to catch the ferry. A tuk Tuk will cost 50 Baht and a motorcycle taxi 20 Baht. Alternatively you can take a taxi from Bangkok straight to Koh Loi Pier which will take approximately one hour and twenty minutes and cost between 800 and 1,000 Baht.
They have a fleet of car and minivans running from the airport to Pattaya and the surrounding area.
You can book with them direct on their website and they will have someone waiting for you at the airport holding up a sign with your name on it. You will need to send them an email and give them your flight number and date and time of arrival. All info on their website
The limo taxis are only suitable for 3 people and cost Baht 1,000 from the airport to Koh Loi pier in Si Racha.
Ferries leave on the hour every hour from Koh Loi pier starting at 0700hrs and the journey takes about 45 minutes. The last ferry to Koh Sichang leaves at 20:00hrs and the last ferry from Koh Sichang to the mainland leaves at 18:00hrs. The fare is 40 Baht per person except for the last ferry of the day which costs 50 Baht.
A Tuk Tuk from the main pier on Koh Sichang to Charlie's Bungalows costs 40 Baht and takes a couple of minutes.
Alternatively you can walk which takes about 8 minutes. From the ferry pier pass under the "Welcome to Koh Sichang" sign and walk towards town. At Seven Eleven take the second left turn and follow the road up the hill (do not take the first small road on the left of Seven Eleven). Keep walking along the road, do not turn left or right just go straight and after about 7 minutes you will see the sign for Charlie's Bungalows on the right hand side.