I went to Self's website, it is awesome, heres the link:
and it says, at your weight, you will lose 400 cals for 30 minutes at a moderate swim pace. That is awesome. But now, bear in mind, that is actual swimming, not just wandering around the pool with your kids and playing splash. Swimming is hard work, but 400 cals is awesome. That is an awesome site for telling you all kinds of activities that take your weight into account, BIG difference in what i burn at 267 versus the average 150 pounder.