...or that is in the water under the earth...

I am puzzled. I get the whole graven images thing - we're not supposed to up anything before God - but the examples of what we're not supposed to worship sometimes confound me. Today, I wonder what exactly is meant by the things that are in the water that is under the earth. I wish I could get into the minds of the Hebrew people, and see what they thought was under the earth, so I could get this passage. Are they referring to cave monsters? Sea Serpents? It's a quandary to me...