You know that monster in the closet? In your child mind, it was whatever is most horrifying. Sometimes, it was just made of darkness and massive size but vague features.
In the sea, there are fish we love to eat, dolphins we love to admire, and whales with all their majesty, but you go a bit deeper and conditions change and creatures take on some very macabre, beautiful and horrifying attributes. They are like all the creatures in our closet and under our bed defined.
Here's just some of the wonders (and horrors) from the sea's closet: