Well, for starters there are a lot of things which you can't smell. For example, pure oxygen has no smell. Neither does carbon monoxide (which is poisonous). That's because your body doesn't have the right "detectors" in your nose. In order for you to smell something, your nose has to have the right sensor for that substance. As far as I know, pure water doesn't have any smell. When you mix it with salt, you can smell it because your nose has a "detector" for detecting salt.

Good question!Water is so important to us that it makes sense (no pun intended) that our ancestors should have been able to "smell it out". Other mammals seem to be able to find water by scent. Let's look at what makes something detectable by smell.

1. Molecules of the substance have to get into the air, and then into our noses. Some things get into the air easily, like gas fumes, other things don't.

2. Molecules have to get a reaction from the nerve endings in our nose. Some molecule shapes get a big reaction (like the ones in perfume or skunk spray), other molecules don't.

3. Our brains have to de-code the message from our noses.

Well, we know water molecules do get into the air, that's what happens with evaporation. Water molecules have a very simple shape; just 2 hydrogens and one oxygen. Perhaps they just don't get a big reaction from the sensors in our nose. The last step is important too. Compared to other mammals, our brains are not very good at recognizing odors. Dogs have 14-20 times more "smell receptors" than we do. They also use a lot more of their brains to process the information, so their sense of smell is about 100 times better than ours. Wolves can smell prey animals up to 1.5 miles away if the wind is blowing the right way. That's nothing compared to the male silk moth. He can smell one molecule of a female's scent in 1,000,000,000,000,000 molecules of air!

So, why can't we smell fresh water? Part of the reason may be that water is not getting much of a reaction from our "smell sensors" due to its molecular shape. Another reason is that our odor gathering and processing system is not as good as that of other animals. Our ancestors may have used logic, memory, communication, and visual cues to help them find water. They also may have paid more attention to smells than we do. You can probably recognize your favorite songs quickly, but all of the music you don't enjoy may sound pretty much the same. If we had spent our lives paying more attention to smells, especially important ones like water, maybe we could smell it better. Of course, we'd still never be as good as it as dogs are.

I found a good web link for those interested in smell:
http://weber.u.washington.edu/~chudler/nosek.html