Charter boat from from the docks at Ranong is a common way to enter Kawthoung and has been used by travellers in Thailand looking for to extend their visa-free stays in Thailand. travel agents in Ranong can arrange this for around 400THB per person including transport to the dock but not including visa fees, for which pristine, unmarked Thai baht or US dollars should be used. Even a slightly marked or torn note will be rejected and change is not usually given at the customs hut on the Kawthoung side.
