Well, there aren't any cars or shopping trolleys poking out, and if you ignore the possible chemical contamination it should be fine. But he's suffered the minor side effect of growing another two heads overnight, but lets not quibble about details.
Rivers are generally much cleaner than they used to be in England. When I was growing up, the River Tees was renowned for being an open sewer, and so much waste was poured into it by the chemical and other heavy industries that lined it that little, if anything, could survive in it. Definitely not for paddling or, god forbid, swimming. It has been cleaned up since though.
The Environment Agency has useful interactive maps that help you find out what various aspects of the environment are like:
http://maps.environment-agency.gov.uk/wiyby/wiybyController?ep=maptopics&lang=_e
I've had a quick look at the rivers and canals in Manchester city centre, and the canals are grade A for chemistry (they don't seem to measure biology in canals, but there are always lots of wildfowl on them so they should be healthy too), whereas the rivers fare less well.
The River Irk, in the section where it runs through the centre, is rated D for chemistry and E for biology (with A the best and F the worst), so not that good unfortunately. But the quality is better about a mile upstream (C and E respectively), so possibly the decline in quality in the city centre stretch is due to historic pollutants still persisting?
The River Irwell, another that runs through the city, is rated D & D in the nearest measurement to the city centre (in Salford).
Both of these rivers have featured in pictures I've posted on this thread, and you can see that they are not exactly raging torrents, so any pollution would presumably persist for quite a while, especially if it is bound up with the sediment in the river bed.
The River Aire in the area of the pub
Fez909 is rated at B for chemistry and D for biology just a wee bit upstream so not too bad.