When it comes to burning calories, swimming tends to fall in the middle of running and cycling for workout efficiency. Running tends to burn the most calories per minute, while biking would burn the least of the 3 triathlon legs.
However, the amount of calories burned in swimming will depend on your stroke technique, and of course the type of workout you are doing. Keep in mind, efficiency is what matters, and training in the aerobic zone, or keeping your heart rate low, is going to allow you to burn more fat than doing a sprint workout (or a swim workout with poor efficiency, wasting energy).
To get an idea of how many calories you are burning in a given workout, check out this chart:
But most importantly, keep this in mind: the caloric theory of weight loss has been debunked! So if you are swimming to lose weight, it won’t likely work, unless you are ultra-disciplined. And even then, it’s not worth it! Instead, pay attention to carbohydrates. If you keep your carb intake to a minimum and replace carbs with fats and protein, you won’t have to even worry about counting calories at all! Instead, you will likely get to your ideal weight quickly.