lots of busses and minivans, just flag them down on sukumvit highway. very cheap lots of busses and minivans, every 30 min. just flag them down on sukumvit highway near a pedestrian overpass or another bus halt. very cheap. 1 hour at most.

Most diving tour on Si Chang is private charter, so you shouldn't have a problem.

You can leave from Si Racha quite late by bus, van or taxi. But since the last ferry from Koh Si Chang to Si Racha leaves at 6 PM, thos determines when you need to leave the island.

They have a bus every hour to sri racha. It takes about 1.5 hours. The bus (minibus) costs 110 baht. On floor 1 door 8 at the airport you can get the tickets (sri racha tours). The people at the information desks can help you. When you arrive in sti racha you can walk to the pier (koh loy pier) or take a tuktuk. From the pier there is a boat every hour for 50 baht. It takes around 40 minutes. I dont know what time the last ferry starts. I guess something around 5 pm or so. Hope this was helpful 😊