I've had turtles for 18 yrs and never had a problem with smell as long as the tank was maintained properly & you do regular water changes. Also, there are things you can do to prevent smelly issues:
I have a very large Adult RES, Sheba (9" long shell), so I feed her in a separate container of fresh water. Any uneaten food pellets (which are made of ground fish, among other nurtitive ingredients) can make the water vile within a matter of 24 hours. For small turtles it isn't a problem since there aren't that many pellets in the water when you feed them & they get eaten quickly, but considering the size of my turtle, feeding in a separate pan of water helps a lot (I discard the water after feeding). I feed my turt lettuce every day in her tank because the lettuce doesn't mucky up the water.
Occasionally Sheba might poo on her basking dock so then there is a bit of a smell from the poop, but once I clean up the dock, everything is fine again.
In my case, full water changes are done monthly (I have a 90-gallon tank). Many of the people on this board do weekly partial water changes instead of the once/month thing. So it just depends on the turtle.
Also, consider the fact that if you have more than one turtle, you will need to work harder to keep the water cleaner. I HIGHLY recommend only having ONE turtle to start off with. You might think that the turtle needs a "friend", but honestly, they do not (are actually very solitary), and having more than one can turn into a territorial war if they start to fight in the tank (a whole separate issue which I won't explain here).
I wash my hands with soap any time I handle Sheba or her water (I have my hands in her tank daily, whether it is to take her out for feeding or to clean something in the tank). While the risk of getting salmonella from a larger turtle is less than from a baby turtle, the risk always exists with ANY aquatic animals. Wash your hands with soap/water & you will never have a problem.

And no, salmonella/bacteria is not an air-born issue. Turtles have their own "normal" bacterial levels in their bodies, just like people do.
