What is the difference between "bromine" and "bromine water" in their reaction with an alkene?
From ChemGuide:


  Using bromine water as a test for alkenes
  
  If you shake an alkene with bromine water (or bubble a gaseous alkene
  through bromine water), the solution becomes colourless. Alkenes
  decolourise bromine water.
  
  The chemistry of the test
  
  This is complicated by the fact that the major product isn't
  1,2-dibromoethane. The water also gets involved in the reaction, and
  most of the product is 2-bromoethanol.


In essence, the electrophilic pi bond in the presence of bromine and water attacks diatomic bromine as usual, forming the usual bromonium ion intermediate. 

After this step, however, we have two competing nucleophiles which can open the ring: bromide ion and water. The water is likely present in great excess, making it the more likely attacker (statistically). 



Of course, the desired reaction below to form a dibromide also occurs. But it is in competition with other reactions such as the one listed above. 



So usually electrophilic halogenation of alkenes takes place in an inert medium such as methylene chloride or carbon tetrachloride. These aren't inclined to act as nucleophiles, so they don't participate in ring opening, leaving the $\ce{Br-}$ ion to do its thing unhindered.