Trains are best but more expensive. I hate buses for longer than 5 hours so I chose train. Worth the extra to have a bed to lay down on and sleep. They trains in Thailand are about the same as Eastern Europe standard. Old and rickety but get the job done.
Ferries go to most of the islands. Those are best sorted at the pier itself. Booking anything in advance always costs more. Sometimes it worth it tho. Ferries I would not worry about. Buses and trains on weekends I would book in advance just to be sure to get a spot.
I replied to your other question as well.
"To Alcohol! The cause of--and solution to--all life's problems." -- Homer J. Simpson
"I didn't get to where I am today worrying about how I am going to feel in the morning." -- Ron White (Blue Collar Comedy Tour)
"I was born to lose and destined to fail." -- Mike Ness, Social Distortion
"Eat healthy, exercise daily, die anyway" -- My motto
"Everyone I know, goes away, in the end...I will let you down, I will make you hurt..." -- Johnny Cash (NIN Cover of "Hurt")
I am traveling around the world until I find something that makes me want to stop. I am an aspiring photographer and hope to find whatever it is I am looking for...