Many people reckon the over night train to Chumpon and then the first ferry out of one of the nearby piers is the go - there are travel agents near the station who will arrange ferry tickets and transfers to the piers.
Others go to Bangkok's southern bus terminal and jump a bus to Chumpon - not so good on account Chumpon's bus station is some way out of town and transport into town was pretty poor last time I passed by. And the southern bus station is some way from where travellers stay in Bangkok.
Private bus companies out of the Khao San Road area have a bad rep - except for Lompayah which runs both overnight and day buses direct to their pier near Chumpon to meet their fast catamaran service. Google the website. I have done their cat and transfers into Chumpon several times although not the full run to/from Bangkok, but I have seen several posts praising the latter.
Go with Lompayah company
#3 RiniRabbit has been a member since 31/7/2012. Posts: 78