At Ranong, you need to go to the Thai immigration office and they will inspect your documents. Once they agree you are fine to be let back in you agree the return fee with the boat guys 200 baht each way is fine - Myanmar is visa free for 14 days but travel is restricted once there. Once you get to Myanmar get off the boat and someone will take you to the immigration which is a few minutes walk away where you will need to pay 500 baht. Go back to the boat, wait for it fill and go back to Thailand.
The whole coast of Prachuap Khiri Khan is a better experience. Samui and other near by islands much less desperate. Krabi, kanchanaburi, Bangkok, all better and less cummy. As you've been everywhere you should know, or maybe you're just a sex tourist.