Koh Sichang is reached by ferry from Siracha. The ferry leaves promptly on the hour and costs 40 baht one way. Alternatively you can hire a boat for about 1,000 baht. Siracha is reached from Bangkok most easily by taxi, about 1,600 - 2,500 baht (from the airport), or much more cheaply by bus. Take shuttle bus from the airport to bus station. Take number 9909 bus to Siracha (80 baht) get off at Robinson (shopping mall). Take tuk tuk to pier to get ferry to Koh Sichang. There are many buses linking the airport and the Eastern Bus Terminal.