Hey Victor,
I know 3 good dive operators with dive trips to the Similans located in Phuket:
Kon-Tiki Phuket Diving & Snorkeling Center, Phuket, Thailand - Divebooker.com
Phuket Pro Dive & Sail o., Ltd., Phuket, Thailand - Divebooker.com
Andy's Scuba Diving, Phuket, Thailand - Divebooker.com...