While not exactly terrifying, the sheer lack of features of this creature give it a kind of chilling nature all it's own. Think of it as the equivalent of the gelatinous cube, only in the ocean.
This creature looks like an underwater iguana and not at all terrifying, until you realize that it can regenerate limbs. That gives the potential for some rather creepy combat situations--let's say a dozen monstrous Axolotl's jump onto the ship and your party keeps trying to kill them only they won't die.
This looks like something out of a horror movie's set design, but it's a real fish. The teeth are so large that they don't fit in it's mouth. Instead they go over the top towards the eyes. Plus it has a blinking light on it's dorsal fin to attract prey.
If that isn't one of the most terrifying things you've ever seen, then you're a braver person than I am. It's the younger sibling of Shelob, only living in the ocean.
It's looks like a cross between a goblin, an orc, and a unicorn. Mean, nasty and a full of sharp teeth this little guy lives so far down in the ocean that the light can't reach him. Oh, and it's jaws can retract.
Blue Ringed Octopus
This creature looks like someone went crazy with the day-glo paint. It's actually really, really small, but incredibly venomous. You get poisoned by one of these guys and your muscles stop working and you die from asphyxiation.
Warty Comb Jelly
This creature is just plain cool. Glow in the dark, and they come in all sorts of different colors. They're also unusual in that their bodies are 97% water, plus they're also hermaphrodites, being able to reproduce with themselves.
The picture gives some idea of the scale of this thing.
Not a trail of fire. Just a creature that looks like it's on fire. I'm sure you can think of many cool uses for this little guy.