That sounds like a great project!
Put only 1mL of water sample on each agar plate (let the ice melt). After you do that, spread the water over the surface of the agar plate with either a sterile inoculating loop or sterile swab. (If you are using disposable varieties of each, use a different loop or swab for each plate.) Remember to use the same volume of each water sample (don't use 1mL of one sample on one petri dish and then 2mL of another sample on another petri dish).
Let the plates air-dry by only partially covering the petri dish (not for long, otherwise, your plates will get contaminated by bacteria in the air.) To shorten the time you have to expose/dry your plates, you can put them in the incubator partially covered, then remove them after they are dry, put the lids back on, and then put them back into the incubator. If the agar is still wet when you incubate it, the colonies will not be circular. Instead, they will form into "splotches" or they will run together with other colonies. If that happens, you can't count the seperate colonies.
Make sure you run multiple trials (2 or 3) of each sample in seperate petri dishes. This will insure accuracy of your data. You could use either boiled water (100 degrees Celsius) or sterile deionized water as a "sterility control" (to insure that there are no bacteria in the agar).
You might want to check the bacterial content of bottled water (a couple brands) as a comparison. Some people drink bottled water because they say that it is "cleaner."
I hope this helps! Best of luck!