Swimming for exercise

I've joined a gym and started swimming for exercise recently (mainly because flat feet and plantar fasciitis made it difficult to enjoy anything else) and I really like it, but I have some questions about it.

I'm not trying to lose weight, but I do want to stay in shape and get good exericse (I used to run all the time...I miss that feeling). I've heard that swimming is actually not great in terms of burning calories and I'm wondering how much of that is true. Personally, it doesn't make me want to eat more than usual, but I've heard that being in the water cools your metabolism so it burns less, and other such things.

Also, I have no idea how long and fast I should be swimming. How much do you swimmers out there swim on a regular basis? (Distances, times, or both). And if you've calculated how many calories you burn, what is the number? (I know it differs from person to person but I'm just trying to get a general idea. Thanks!

I swim 30 minutes/1600 yds 2-3 times a week.   However, I am switching pools soon, and I will be able to swim 1hr/3200 yds 2-3 times a week. 

As for calories, according to swimming.about.com, if you multiply your weight by 3, that gives you an estimate of the number of calories that you burn swimming 1 mile (1760 yards or 1600 meters).  However, this is a rough estimate at best, and it varies greatly based on how fast you as swimming, how fit you are, etc...