The reason Swim.com does not have calorie tracking is that calories burned are extremely difficult to accurately estimate for each individual person using just body weight and it is only with an accurate heart rate monitor combined in calculation with individual body statistics (height, weight, gender, age, BMI, etc) that you would get a fairly accurate statistic.
Many of the swim tracking devices that come with a heart rate monitor, such as the Garmin Fenix or Forerunner series, are sold with a chest strap that transmits on Bluetooth frequencies that do not work in water. Most wrist-based HR watches have also turned off HR monitoring while doing a lap swim workout because the water interferes with measurement taken by the optical hardware. The exception here is Apple and Samsung - they did not disable wrist HR for lap swimming activity tracking. However, water droplets that get trapped between the wrist and the back of the watch refract the light and usually cause higher than normal readings. For example during several of tests with an Apple Watch we were able to get bpm readings of anywhere from 205-212, which is too high.
There are a few devices out there that will accurately record HR while swimming, such as the Swimovate Poolmate HR, which has a chest strap that transmits on a 122KHz frequency. Garmin also has a HRM-SWIM strap that stores HR data inside the unit and then syncs it to your workout later. These devices, however, are currently niche products and combine for a very small share of the lap swim tracking device market. For this reason, our small development team has decided not to pursue adding data from these devices at this time.
One thing we have done though, is added a metric for average heart rate over the entire workout for Apple Watch only. This metric, which is visible by viewing your Swim.com workout on the Apple Activity app, is averaging all the measurements over the entire course of the workout and a few readings skewed high will not affect the data as much. One thing to note is that we have come to find through many rounds of development processes that this HR data is more of a ballpark figure and not 100% accurate, and therefore will likely not produce accurate calorie expenditure data.
In short, because of the current market conditions, inaccuracies in data, difficulties involved with the data collection, and because it is not a metric that enhances the friendly competition aspect of our product (leaderboard), we have not yet incorporated calorie data into Swim.com.