Most divers will agree that you are the most dangerous underwater creature—at least to yourself.
Humans aside, it depends on where you dive. Creatures that actually attack you are quite rare. In over 20 years of diving—over 900 dives in a dozen countries—I’ve never been attacked by anything but jellyfish. And most jellyfish aren’t really dangerous—especially if you always wear a full wetsuit and hood, which I do.
What to worry about is creatures that can defend themselves with deadly force if they think you’re attacking them. These are also few in variety.
The only kill-you fish I know of is the stonefish.
The only kill-you reptile I know of is the sea snake.
Among invertebrates, there’s the blue-ring octopus and a cousin whose name I forget, and cone shells.
Every year some fool diver picks up a cone shell—they can be pretty big as shellfish go, and most are very pretty—and tucks it in his wetsuit to take home to his girlfriend. Instead he gets a Darwin Award. Cone shells have one of the most potent neurotoxins known. No antidote, and usually fatal. Even if you pick one up by the very back of the shell its harpoon is long enough to reach around and nail you.