They look scary, especially in that an eel's mouth open and closes in breathing, looking like it wants to take a bite out of you. It's easy to avoid a bite from an eel. And, if you are spear fishing, don't dangle your catch close to your body. Use a float, so if an eel decides to snatch on of your fish, it won't grab your hand by mistake. Eel's teeth are concave, so if you do get bit, don't pull away.
Jelly fish are another creature to stay clear of.
Sometimes they come in with the winds and after storms. Most of the popular beaches have lifeguards who post signs to warn everyone to stay out of the water. Unless you are allergic to them, the sting is about equivalent to a bee sting. If you have any signs of an allergic reaction, get medical help, fast. When we first moved to the Big Island, we saw this warning on a city sign in a vacant lot.