Why do you get that 180 bond angle between water molecules?

Right this is difficult to explain without a board pen and some pointing but here we go.  

The 180 degrees I am referring to here is for the H that is covalently bonded to an O on one side and Hydrogen bonded to an O on the other (see picture). 

This hydrogen has a covalent bond on one side (i.e. a bonded pair) and a hydrogen bond on the other. Now, technically the hydrogen bond is not a bonded pair of electrons but the oxygen that the hydrogen is hydrogen bonded to is a big ball of negativity (imagine a fat dementor) that behaves much like having a pair of electrons on that side too. So effectively that hydrogen has 2 bonded pairs, and what shape do molecules with 2 bonded pairs take...straight line. 

So there we go, that is your answer. The actual answer in the exam goes along the lines of...

"...because it has two bonded pairs of electrons and to take the position of minimal repulsion the electron pairs  get as far away from each other as possible which is a straight line."