there are heaps of great bars/pubs around Waterloo. My favourites are:
Laughing Gravy - Blackfriars Rd
Windmill - The Cut
Bar Citrus - The Cut
Really close to Waterloo is the Firestation, always busy.
If you want to be closer to the river try the NFT bar or the bar on the OXO tower, bit pricey that one.
The bar at Gabriel's Wharf is good fun but you get served in plastic glasses which is a bit nasty. There are also a couple of chain pubs around like HOGSHEAD and ALL BAR ONE and Bar Med down by London Eye.