Easy enough for runners or other load bearing exercises.  But in swimming (and I suppose other activities) there are many variables.  Is it possible to come up with a meaningful average given:

• the mass of the swimmer
• the speed of the swimmer; and
• the resistance of the water, which will vary depending on

1. how much of the swimming stroke is out of the water and in the air (arm recovery and also stroke dependent, i.e breaststroke, butterfly, etc);
2. the streamlining efficiency of the swimmer, which will affect the resistance of the water
3. how long a swimmer can maintain a given efficiency (tiredness during the hour will impair the ability to stay efficient)
4. the temperature of the water