The word meaning "someone that tows" would always have the 'o' sound. However, I don't think it's often used, because usually, you have a different type of thing towing to the thing that's being towed, so you can just say 'car' or 'tractor' or 'tug' and it's clear from the context. It would be the same with 'shower'... but again, we don't often use that to mean "someone who shows".

Tower (building) is clear, for the 2nd I would go for 'tow'wer', as in 'to tow'. Most languages have this issues were the written script is not consistent with the phonetics in some situations.

The two words have clearly different pronunciations. The verb 'tow' rhymes with the adjective 'low'. So if you want to make a noun from 'tow'= 'er' it would rhyme with 'lower'. That said, I have never come across such a noun. If you want to refer to a boat which tows another boat you would use a specific term for that type of vessel.

...and my curiosity is satiated. ;) This looks like sloppy editing - the "thing that tows" definition doesn't even turn up in other dictionaries. This link has both meanings, but is clear about the pronunciation: http://dictionary.reference.com/browse/tower

http://www.yourdictionary.com/tower This is where I came across the word ( tower ) having two senses but one pronunciation .