hate to disappoint you but all what cool luke has said is the painfull truth,most of the dive master`s and instructors are free lance,i kind of know an instructor who does free lance work there and he has told me yes at peak season the is a demand for dive instructors but not so much the dive masters.
Even then your there for the life style and you just make enough to get by,last time i spoke to him he had only made 10k bht and remember even though we think it is reasonably cheap to live out there we are still being paid western money not Thai money.
As for your visa you can obtain a double enty visa, but what ever you do dont call it a six month visa , for some reason ive been told they dont like it being called that and it can rub people up the wrong way.
regarding your dive instruction the is a company which advertise such programmes ,you can look them up at
www.pjscuba.com
they do insist on payment in advance, i have looked into doing it myself,but i have a an ok job in the uk but with good benerfits,
if you go for it good luck to you and i respect you for following you r dream.