Sadly (in this case) blacktip reef sharks are "Timid and skittish, the blacktip reef shark is difficult to approach and seldom poses a danger to humans unless roused by food."[1]. Happily that means you can swim near them safely, they hang out in Ao Leuk (I think?) on Koh Tao as well.

[1] https://en.wikipedia.org/wiki/Blacktip_reef_shark