Hi Neil,
I am very envious of you - what a great way to spend Christmas!! Underwater - and at Similans!! I am sure u will have lots of fun.
OK here's some of my fav sites at Similans/Surins:
Beacon's Beach - for a check out dive, this is really nice. Great site to work on weightage and buoyancy, and trust me, you will get distracted by all those pretty little fish, and the large varieties of fish and marine life.
East of Eden - another great dive. This is the home of a huge resident moray eel, who is super friendly. Also saw mantis shrimp, trumpetfish, different species of triggerfish (lovely clown triggerfish), garden eels, octopus and many different reef fish.
Hideaway - this one is special to me, coz here's where i spotted the black ribbon eel, and quite by accident! This is also where we saw the bizarre looking seamoth. Lots of garden eels and lionfish as well.
Bird Rock - i loved this place for its exciting swim throughs and fantastic underwater scenery. Very strong currents which made for an exhilarating ride.
Christmas Point - another fantastic dive with super strong currents, exciting swim throughs and fantastic underwater scenery. Not many divers fancy diving in such conditions though, so it's really up to your individual preference. This is where we discovered an air pocket at the ceilings of one of swimthroughs, school of barracudas and the fierce tuna.
Richelieu Rock - simply fantastic. The only way to top this would be to spot a whale shark or manta
There were tons of beautiful lionfish, scorpionfish (careful not to touch anything!!), cute little white-banded cleaner shrimp, banded boxer shrimp, Durban dancing shrimp, transparent little partner shrimps, - so many different types in huge quantities! Also spotted was the pipefish, school of barracuda, boxfish and pufferfish (i saw a cute little yellow spotted one which i fell in love with), porcupine fish and the many varieties of reef fish.
Wreck Boonsug - our last dive which was somewhere near to Tap Lamu. The viz was not too good, but this site is famous for tons and tons of lionfish - they were simply everywhere, tons and tons of porcupinefish - my favourite fish, and tons and tons of cute little moray eels living on the wreck.
My friend who has dived Phuket gave the thumbs up for Anemone Reef. I've heard that Shark Point is OK too, while King Cruiser should be quite an interesting dive though viz is not good. You might want to consider Racha Noi, which is supposed to have very dramatic underwater scenery, and possible big-fish encounters. However, this place has some challenging and unpredictable currents, hence more suitable for experienced divers.
You might have to manage your expectations, coz it seems that Phuket diving and even Phi Phi diving is not as good as Similans. So if you dive Phuket/Phi Phi after Similans, you might feel a little left-down. I will be diving Phi Phi in Feb, and will keep u updated on the diving there