Originally Posted by John Drohan
Also, and please correct me if I'm wrong, heart rate is also a factor in overall body cooling. So it's also going to be a function of ambient temperature. It would make sense that swimmer's max heart rates would be lower due to their environment. Bicycling would be lower than running due to higher convection-based cooling.
Yes I believe you are correct. I believe horizontal body position is also important in swimming as is water pressure (although slight).