why do i measure a potential difference when I put my two electrodes in a water bottle?
This is common, and is because the two probes aren't exactly of the same metal.  They may have started out that way, but over time each was exposed to a different history of corrosion, sweaty fingers, and the like.  The result is that the two probes no longer have exactly the same electro-chemical potential.  

Put another way, each probe in the water is acting like half a battery.  Since they aren't identical, they don't produce exactly the same voltage.  The difference between the two half-battery potentials is what the meter is showing you.

Instead of one of the probes directly in the water, try a different metal and connect the probe to that metal out of the water.  That should give a larger difference between the probes.  You can also try other conductive substances, like a graphite rod.