Passenger boats leave regularly from the island temple of Wat Ko Loi, the trip takes just over an hour and the island is well worth a visit as there are many things to see and you are able to walk to all the places if you can bear the heat. Try get a place on the top deck of the boat as it is cooler and you will be able to enjoy the view.
There is a shopping mall located in the main street and next door is a minibus office that depast to Bangkok every 30 minute for just over 100 baht. This is also where the buses stop too. We got on the minibus which dropped us near the airport then we transferred to a local bus.
In some of the smaller towns in Chon Buri province, there are no tuk tuks to be seen but Si Racha is firmly in tuk tuk territory. Expect to pay about 50 baht between two points within the city and 100 baht to get out to the Tiger Zoo. As always, negotiate the fare before you depart.
Si Racha is located on Sukhumvit Road, which runs from Bangkok and continues on to Pattaya. If you want to get there from one of the three towns listed above, just stand out on Sukhumvit and wait for a large bus heading in the right direction.
All the buses I came across were air-conditioned but I'm sure there are a few that are not. The fare is between 50 and 80 baht and the journey takes about 30 minutes from Pattaya or Bang Saen and 45 minutes from Chon Buri.
The popular way to do sightseeing around the island, take this transport, locals call it "Sky-Lap", it has similar look as TuK Tuk. The driver took us to every attractions in the island for 250 Baht.
It takes 45 min ferry from Koh Loi Pier in Si Racha to Koh Si Chang. The round trip ticket is 100 Baht, leaving every 2 hours.