Bit surprised to see a Student Union bar on this site. As student bars go, Waterfront Bar is pretty good. OK, so the selection of beers and drinks are generic (but cheap!), and the food fitting the 'bargain basement tummy filler' category.
The thing that makes Waterfront Bar unique is the truly spectacular views across/down/over the Thames. You can see the Docklands and Westminister from the same window. Not many places in London offer these kinds of views (even the Oxo tower is inferior).
As civvies can only get in if they're with a King's student you can't just stroll in as you wish. So if a King's student friend asks you to meet them here for a couple of drinks, there are 2 reasons why you shouldn't dismiss it: 1) you'll actually get in and 2) there really are much much worse places to drink.