There are lots of dangerous underwater creatures. The most dangerous is not the shark, as most people imagine. I have dived with sharks many times, and never had the slightest problem. I've even touched several sharks. Probably the riskiest encounter I ever had with a shark was during a night dive in the Red Sea. When we got out of the water my buddy told me that during the dive he had shone his torch behind us, and seen that we were being followed by a large shark. The consoling thought about that was that it would probably have taken him first.
Sea snakes are supposed to be six times as poisonous as the most venomous land snake. Yet we very rarely hear of anybody being bitten by a sea snake. I've dive with them several times and really enjoyed the experience. I once went on an expedition to the Swain Reefs, off Australia, specifically to dive with sea snakes. They're very curious creatures and will come up to you to see what you are and will twist themselves around your body. As long as you don't panic and try and strike out at them there's no cause for anxiety.
The crowd of thorns star fish destroys coral reefs. They're been spreading through the barrier reef for many years and they leave devastated areas of white coral in their wake. I used to have the habit of stabbing them with my dive knife whenever I saw them. That stopped when I got a bit of thorn in my finger and was quite ill for several days. I was lucky, I heard of somebody who had to be airlifted off a dive boat to an Australian hospital after they had the same kind of accident.
I once had a chance to dive in a lake that had a crocodile in it. I have always been a bit wary about crocodile and wasn't entirely sorry when the expedition was canceled at the last minute when somebody had an accident.