About a detective who looses his partner when he falls through a man hole cover in a flooded city. His partner becomes a mutated creature and goes on a killing spree.

Might be this one? Except it's not his partner that is the creature, the creature simply ate his partner in the past.

Split Second (1992) - Plot Summary (http://www.imdb.com/title/tt0105459/plotsummary)

http://en.wikipedia.org/wiki/Split_Second_(1992_film)