I did a border bounce from Ranong last Sunday. The immigration officer on the Burmese side told me I could spend up to two weeks in Kawthaung, but I had to stay within a 20km radius of the town. If I wanted to travel further into the country (including the nearby islands) I would need a visa (to be obtained online or at a consulate), but it is possible
the stops are literally a minute, you don’t even get out of the boat, the driver runs up to the window to show your passport and off you go.
The borders with Cambodia are roads, so no boats involved, but from what I read in this group and others, the Thai immigration officers can be very difficult there