Definitely DO NOT “speed swim”. You will not out swim them, and actually attract the shark to you.
Stand in the water with your feet towards the bottom, hands and arms across your chest, with your hands on your shoulders… So your arms are tucked in, not flailing.
Head/Mask down to watch the shark at all times while “walking” back towards the boat, using your flippers to “walk”.
If the shark approaches, face it standing position, this confuses it, since most food sources swim vertically not horizontally… Hopefully he just investigates and swims on.
Scream through your mask to confuse it, and prepare to fight if it gets within range to do so.