it was a PADI course and part of the course rules were that you had to be able to swim 100 metres. At Koh Tao that was done by swimming out from the shore 50 metres and then back 50 metres. It could be freestyle, backstroke, breaststroke, even dog paddle, as long as it could be done non stop, and it was not an extra rule, the same rule of being able to swim 100 metres also applied at the PADI dive schools in Sydney too. It may not be a rule now based on your comment, but back then it was definitely the case. I wasn't a strong freestyle swimmer, so I did it breastroke, they even gave me the option of being able to wear fins but it had to be 200 metres.