Here's why:

* Sulfur hexafluoride is a gas: It exists as a gas at room temperature and pressure. It doesn't interact with water in a way that would make it disappear.

* Invisibility is a complex concept: Making something invisible usually involves manipulating light waves, not simply mixing substances.

* SF6's properties: While SF6 is denser than air and has some interesting properties like slowing down sound, it doesn't have any ability to make water invisible.

Let's talk about how invisibility is often portrayed in fiction:

* Refraction: In some fictional scenarios, invisibility is achieved by bending light around an object, making it appear as if it's not there. This is based on the concept of refraction, but it's far more complex and currently impossible to achieve with real-world materials.

If you're interested in learning more about the science behind invisibility, you can research concepts like:

* Metamaterials: These are artificially engineered materials that can manipulate light in unusual ways.

* Cloaking: This is a theoretical concept where light waves are diverted around an object, creating the illusion of invisibility.

Remember, while invisibility is a captivating concept, it remains firmly in the realm of science fiction for now.