Blimey, that makes me think! Firstly as a 24 inch diameter pipe it would be too big to bury in the deck of the bridge, hence the decision to put it in a tunnel. When British Gas was arranged in regions, there was a South Eastern Region (who I worked for!) and a North Thames Region. For some strange reason, we went over the river to cover Hampton, Teddington etc - hence the need for the pipe. It used to run right along the barge walk and over the years had a problem with stress corrosion of the ductile iron bolts that formed the joints - the solution was to purge the pipe of gas and send people up the inside, sealing each joint. It was a filthy job and it fell upon me to crawl along afterwards (many hundreds of yards between shafts) and inspect every joint. When I look at myself now I think it would be a bit more tricky to fit in a 24'' - a vision Winnie the Pooh getting stuck comes to mind! I actually worked with a guy who turn round in that size, although how he had the nerve to try it the first time always mazed me!
Nothing to do with boats at all though - sorry!