This (the point marked on the map) is probably one of the best snorkeling sites in the world.
Here you will see huge turtles, clownfish, pipefish, boxfish, trumpetfish, frogfish, cuttlefish, squid, crocodile fish, barracuda, stingrays, morays; frankly the list just goes on and on. We accessed this site from the Scuba Junkie Jetty (we'd stayed in Semporna and booked a day trip with Scuba Junkie).

• Ease of Access: Straight off the beach/jetty
• Hazards: Boat Traffic/Coral/Terrorists/Pirates
• Visibility: Variable
• Best time to go: March to October
• Water Temperature: 26 - 32 C°
• Highlights: Turtles, Vast amounts and variety of fish, coral

I love this place too!

If I could be anywhere, snorkel anywhere (although in fairness there are so many places I haven't been, but I suppose ignorance is bliss!) then it would be in Mabul!

I dream of going back (I WILL go back)

I have never seen such an abundance of life. The turtles are huge and seem so used to people that they abide you (for a while until a few flicks of their fins and they are off into the blue). I did my best to hold my breath and swim underwater with some of the big turtles and the experience was just amazing. Although I was put to shame by the free diving locals who worked for the tour company - they stayed under for so so long.

Along with all the other marine life you mentioned, I did see a number of stunning Lionfish just hanging around the jetty too.

https://www.youtube.com/watch?v=btEdarGmrKE

Our video of swimming with turtles in Mabul