I can't begin to explain anything if you are going to base your fears on Hollywood. Even if you did get left behind you can always get a ride back with one of the swarm of other boats around. There are also inumerable risks from conspiracies, alien invasion and, if you happen to have a third nipple, being shot by James Bond...
Here'sthe PADI guide to Thailand:
http://www.padidiveguides.com/THindex.html
and Phuket in particular:
http://www.padidiveguides.com/TH/And.../andaman.html?
http://www.padidiveguides.com/TH/And...st.html?d_id=7
Many dive shops in Patong will sign you up with a dive operation to take any course from Open Water upwards and they are a good deal cheaper, and warmer, than doing them in a reservoir in Staines.
Warm Water Divers (
http://www.warmwaterdivers.com/) are a friendly operation, on Ratuthit Road (smaller office on Beach Road) a little way past Christin's and they can book you on courses and day-trips with Calypso Divers (
http://www.calypsophuket.com/) who are a small and friendly company whose boat Greta serves excellent pancakes on the way back from dives and has a decent clean-air policy of restricting smoker scum to the upper deck only.
All operations pick up from hotels in Patong, usually around 0730, and run boats out of Chalong Harbour (half-hour ride by the time everyone's been collected).
With PADI you can do all your theory and pool sections of the Open Water certification in the UK andelect ot do the open water bits abroad (warm) at any PADI centre. If you're anywhere near Hammersnith, try Scub Training Ltd, Fulham Road (
http://www.e-scuba.co.uk/).
Cheers,
K.