[espadrine]

> Why would it be hard to determine that we're intelligent?

Underwater sounds can come from a wide range of sources, including tectonics, natural events and animals. Those did not create some sort of apparatus. Yet, they can have a similar signature.

It is not even a hypothetical situation. A SETI lab spent a lot of time trying to figure out what a particular signature that their telescope picked up was, only to discover it was caused by their microwave oven: http://www.independent.co.uk/news/science/seti-the-hunt-for-....