If you google, you should be able to find calorie burning estimates for all sorts of activities including swimming, but as with walking, running, and most exercises everyone's perception of intensity is going to be different, and your body weight and metabolism are going to skew those numbers, anyway, so here's my simple guideline.
How hard is it? Do you do it until you're breathing harder or a little out of breath? If you get out of the pool before cool-down can you tell that you are sweating. If not, try it. Put in your laps, get out of the pool and walk slowly around the pool or to the locker room. Are you sweating? Breathing a little hard? Do your legs feel a little sore or "rubbery." Can you feel it the next day?
For me, swimming is the only way I can really get into an aerobic heart rate. On land, pain or my asthma will sideline me before I can barely get started. In the water, I can work at a much higher intensity, so I can burn a lot more calories in the water than I can on land. I don't notice it "in the water," as much as after I get out. If I do cool-down in the water, I don't notice that I'm sweating as much (my face will be sweaty, but I just keep dunking myself to cool myself down). When I get out of the pool, that's when I notice how intensely I was working. If my legs are a bit wobbly and sore, I know I did good. If I'm a little sore the next day, I know I did REALLY good.
These are the guidelines I've always used (even when I was much thinner).