We went to Koh Lipe in October a few years back, so the tail end of 'wet' season when you'd normally expect the most rain on the Andaman side of Thailand.
Stayed on sunrise beach on the east and dived with Adang Sea Divers. It's a nice island and weather was absolutely fine. The only rain I remember was a thunderstorm at night once, which was pretty spectacular!
Only did local dives from a longtail boat. All very chilled and easy, but nice, especially Stonehenge which has some interesting rock formations (hence the site name) covered with white and purple soft coral.
I believe there are some more interesting and challenging sites further away from Lipe island, eg 8 Mile.
One thing to bear in mind though, the sea can be pretty lumpy during that time of year (I guess that goes for June too??) and boat trip from the mainland was awful! Most people on the boat seemed to end up throwing up. Luckily my family managed to keep it down but it was the worst boat ride of my life! Mind you the ride back a week later was smooth and very pleasant, so I guess it depends on luck!
We stayed on Koh Tao's east coast 3 years ago. Very quiet and chilled. The west side of the island is more lively, but the dive sites are busier!!
Dived with Calypso divers who were great! Only did local dives along the east coast which were nice and quiet but nothing mind-blowing. As mentioned above, there are some top dive sites within reach as a day trip from Koh Tao, eg Chumphon pinnacle.
So maybe worth a look at Koh Tao too.
