Yes, I can confirm that today we saw a Whaleshark at Hin Muang!!!
Purple Rock (Hin Muang) is my personal favourite dive site and it is superb when the big stuff is there, because it is very hard to miss it. The site is completely submerged, giving an excellent viewing area above the reef to see any Manta Rays or indeed Whalesharks.
I am totally gutted, wished I had been on the boat ;-(