Welcome! Someone please correct me if I'm wrong, but I think what happens is that the 1800 is based on your overall lifestyle -- on the days that you do water aerobics, you're simply burning the extra ~250 cals, which will help you lose weight a little bit faster.
Or, you could add the extra ~250 cals on those days and your "burn" every day would be the same, but I think if you're supposed to be on an 1800 diet regardless of activity, maybe you shouldn't do that ^_^ In that case, the exercise if just helping you lose weight a little more easily.
Goodness, that answer might be confusing!
Female, 28 years old, 5'4 1/2" tall
Starting weight 1/4/11 = 215.2 lbs.
Weight 9/3/12 = 164.9 (net: -50.3 lbs.)
Current mini-goal: 160 lbs.
Next mini-goal: 150.2 lbs. <--- Official "Healthy BMI" weight
Estimated final goal: 130 lbs.
"You don't have to change your life today. You only need to change your day today."