One of the problems with online calculators (including Spark) is that there is an implicit assumption that calorie burn is proportionate to body weight. This holds true for most activities (eg. walking, running, aerobics), but there are exceptions.
And water aerobics is one of those exceptions. Your bodyweight is largely irrelevant, as your weight is supported by the water. What matters is how vigorously you are moving against the resistance of the water. So Spark will underestimate the burn for lighter people, and overestimate the burn for heavier people.
It is possible for two people in the same water aerobics class to burn different amounts, as someone can be doing the movements vigorously, while someone else does them fairly gently. And as this happens underwater, it is hard to tell the difference.
As a very rough guide, a reasonably fit person working out vigorously will probably burn around 10-12 calories per minute on a sustained basis.
The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.
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