Oh... ahem... err... nothing. No, there have been divided opinions among locals on the island about just how big the tourist industry is becoming I think, and whereas some people have capitalised on this, others have not, leading to resentment. I think a lot of the businesses are run by crime families too. There was also that high profile rape and murder.
BUT BUT BUT I'm sure that horrible crime was a one-off - they caught the two responsible didn't they. And I'm sure you will find your holiday heavenly and peaceful.
I don't know about diving on Samui, I bet you can do it. As I said, if you could duck off to Koh Tao for a day or two - it is well known for diving as it is marine reserve. Used to have to get a boat to phang-an, and then a 4 hour boat to koh tao, but i bet you can go direct in half that time now. I'm sure your bf would know about where to dive anyway.