Weight loss over summer?

Over the summer I was hoping to lose about 20 pounds. I've been trying to reduce my caloric intake and eat better (healthier, smaller portions, less snacking, etc.) I've also increased my exercise seeing that I have a lot more time without school and I also swim laps for an hour a day three days a week (not much, but still) Anyway, I was wondering if 20 pounds is a realistic goal to reach over the summer (two full months plus about two weeks) or am I aiming too high? If it's not realistic, what would be a better goal. Summer is the easiest time for me to lose weight, last summer I was able to drop about 17 pounds (I didn't feel much thinner so it was probably mostly water weight but I'm just going by what the scale says =/)