Moray Eels can be very dangerous when provoked.
In 1948 Vernon E. Brock, former Director of the Hawaiian Institute of Marine Biology of the University of Hawaii, was collecting fishes at Johnson Island (about 1000km WSW of Oahu). He speared a large moray which writhed up the spear and pursued Brock who had dropped the spear and was swimming away as fast as possible. The eel caught him and Brock was bitten on the elbow after lifting his arm to protect his face.
Brock was released by the eel and swam back to the boat, where a torniquet was applied to slow the profuse bleeding. A navy doctor spent two and a half hours sewing the wounds before Brock was flown to Tripler Hospital in Honolulu. Brock spent a month in hospital and then a further 2 months before he had regained reasonable use of the arm.
The eel responsible for the attack on Brock was most likely Giant Moray, Gymnothorax javanicus. It was estimated to have been over two metres long.