[Jack]

I find all the answers here are pretty useful and i will also add 1 tip to the list.
If you must surface while a shark is in your immediate vicinity, for example, to low air supply: move slowly and deliberately, never loosing sight of the shark. Swim directly to your pre-planned exit site. Avoid a long surface swim, which is tiring and may leave you vulnerable to attack from below; when you arrive at your exit point, exit the water smoothly but efficiently. Incidentally, if you spot a shark from the surface and choose to dive with it, enter the water quietly, using a controlled seated entry rather than a giant stride entry - not only are you less likely to frighten away any nearby shark, but I have seen sharks react with great excitement to the commotion caused by such a spectacular, noisy entry; in an excited state, a shark can become dangerous in an instant.

Above all, enjoy the experience. You will almost certainly remember the encounter long after the shark has forgotten you.