What Am I?

Clue 1: I am a type of ocean animal that has no bones and no shell.

Clue 2: Almost all of me is made
of water.

Clue 3: I am called a fish but I am not
a fish.

Clue 4: Sometimes I am almost invisible. You can see straight
through me.

Clue 5: My soft body looks a lot like
an umbrella.

Clue 6: I have lots of long stinging tentacles hanging from my body.