A fart smells worse in the tub because the fart molecules (which are basically gaseous poo) percolate through the water and form chemical bonds with the H20 molecules before reaching the air.

The new fart/water molecules reach the air/water line in the tub and release into the immediate atmosphere as a noxious water vapor + fart molecule. This molecule is heavier than the normal "non-water" filtrated fart and tend to linger in the immediate area longer than their lighter and more quickly dispersing air-based fart brethren.

Additionally, the water content in the tub-originated fart molecule tend to hit and connect to your nasal receptors with greater stubbornness than normal farts which also enhances the perception of stinkiness of the tub fart.

Even though they smell bad, it is perfectly healthy and natural to fart in the tub... Be careful that you do not inadvertently push out the "escape artist" turd though.

Happy Tubbin'!