@brotherbloat If you get heart rate during swimming (I believe you use auto-tracking because Fitbit disables the HR sensor on the majority of devices during swimming) then it isn't accurate. Simply, Fitbit devices are not made for tracking HR in the water. Not many wearables have even acceptable accuracy during swimming. HR may be just ignored. I did tests with Ionic, Sense, Sense 2 and recently Pixel Watch forcing them to track HR in water HR is there but it's all rubbish - not matching chest strap data and not even matching workout structure (taking accounts intervals and intensities).
I don't know how calories are being calculated by Fitbit. Swimming, in most cases, is a low aerobic activity. Different strokes will have different energy needs. For example, the butterfly is known to be the best "calorie burner". Fitbit recognizes styles but from my experience - it does it all wrong (for me, it's always backstroke detected which I rarely even do). Yet, I don't know whether stroke type contributes to calorie estimation.
90kcal for 25min doesn't look too suspicious and it's still probable (500m in 25min is a leisure-like pace). Looking at my own swimming tables, swimming with a chest strap gives me 80 - 90kcal per 500m (freestyle), so maybe it's about right (and I swim rather slow 2:30/100m). If you swim 500m within 25 minutes or 10 minutes that won't matter much as you still fight the same draft force and water resistance. The only difference is more muscle fatigue. Compare it to swimming with hand paddles. When I do intervals with paddles, I can swim faster (stronger pull) yet more muscle fatigue kicks in. But eventually, 500m ends up with about 90kcal just not within 12 minutes but less.
In general, non-competitive and leisure swimming are not the greatest calorie burners (it may be less than walking).
