I've had a few encounters and they've been mostly while spearfishing. A few sand sharks and one smallish (6') tiger shark. I've had to poke a small 3' sand shark twice with my spear gun as he wouldn't leave me alone and was trying to go after my fish on the stringer. Most sharks will turn around and swim away.
The biggest shark I've seen is a hammerhead and he was at least 12' and was playing around with half of what looked like a 100lb tarpon. I couldn't imagine being in the water with that shark. It was a surreal moment watching him swim around the boat and not scared at all.