All equipment leaving Earth is sterilized. The thoroughness of sterilization varies from project to project, instrument to instrument, and the agreed to HW & Planetary Protection Plans. It helps ensure that equipment is not giving false readings. Serious money is also laid out for HW & Planetary Protection (there's a whole division of probably 150 people at JPL) and sophisticated equipment used to test for contamination.
However, if there was deviation from the original Planetary Protection Plan [1], it was likely to execute some other test, timing/scheduling conflict, or some other "administrative" reason.
And with respect to finding water on Mars, why would you send your robot to a polar cap or even a "lake" with visible/detectable water? If you're attempting to prove that water and water flow processes exist & are active on Mars, it's probably better to test a fringe area, hence, the 3.3m requirement.
[0] https://news.ycombinator.com/item?id=6809134
[1] http://www.space.com/13783-nasa-msl-curiosity-mars-rover-pla...