The most dangerous creature you might find are the Bull sharks, but these are RARE, found only at 1 or 2 dive sites, and even then don't care to bother you. There are nurse sharks, but these can't really be called dangerous. You will see barracuda and eels, but again - just keep your distance.
There are charters that let you snrokel with rays & sharks. We did this and had fun. The Stingrays swarm around you and rub agains you becuase the guides chum the water. We did not see the sharks that day, becuase the weather kept them on the other side of the reef. This was all very safe: avoid stepping on the rays, as they have an infectious barbed spine. Don't stick your fingers in their mouths and you'll have a ball.
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