OK. This happened to this girl who was snorkelling with me. The poor lass was wearing a nice 2 piece when in the water. Schools of fish swamped her many times. As much as she enjoyed their attention, she later complained that she got bites from the fishes !!!
So, to avoid getting bitten, wear a t-shirt and then, go snorkelling :-)
Well, not really a danger, but on the small Ko Kradan beach there were some big crabs a bit menacing.
One of them tried to pinch my toes several times. These crabs are black and leave by the rocks; they are different from the usual very common (and much smaller) sand crabs that dig holes int the sand on most tropical beaches.