There is still no official info regarding health insurance so no one knows when or if that will happen. But I do think ppl should have an insurance, for me its common sence.
Do you have an extension of stay? Since you say 4 months left I guess that is the case.
Then you have to cancel that and leave the country.
Your company sounds like very unserious but even though its your responsibility to know the due date of your work permit and you shouldnt have kept working after wp expired.
Your dive center can help you, with the amount of foreign dive instructors on the island Im sure your dive center gone through this process many times before.
You will need to leave for the NON-B, not heard anyone managing to change visa exempt to NON-B in Samui.