Yes you need return flight 20 000 b and proof of residence for Thai immigration if you don t have you ask the local agent he will arrange everything for you for 1800b including the boat 2 ways for Myanmar - it s not a ferry but long tail boat
Another option is to go to the Adaman hotel pier the price is 2500 all including by speed boat
For Myanmar immigration you need copy of your passport and 10 USD ( not damaged ) and tip to the guy who will welcome you
You will get 15days visa but you have to stay in a area of 10km around Kawthaung