Gilmour claimed he played a lot of bass on Pink Floyd material in the '80s when 1) he had gone through a messy musical divorce from Waters and was 2) quite partial to cocaine. I imagine a half-joked, and highly arrogant "I played bass on that!" has somehow become the written word.
Waters is good on all the bootlegs from the '70s. There is no way anybody else, barring Snowy White from 1977 on, was playing the bass other than him. Live he had to recreate those lines. What I find interesting is his use of melody. Not only does he use a lot of arpeggio patterns and phrases, he often plays lines that feature a slightly different rhythm or pattern to the drums, such as a swinging bass line over a straight beat. His use of octave leaps is also pretty unique and interesting. Bootlegs tend to show this off best as he was often quite loud live.