sit and kick wrote:
check your heart rate maybe?
It is very hard to get the same heart rate when swimming as for running or skiing or cycling.
I Because you are horisontal so the blood does not need to be pumped from your legs
II Because smaller muscle groups (arms) are those pulling you forward.
Cycling HR is lower than running HR, as well. You use your core way more with running, in cycling all power needs to come from the legs. So for most people, running max HR is ~10 bpm higher than cycling.
For swimming - yes horizontal position is one of the three main reasons why max HR in swimming is ~15-20 bpm lower than with running or XC skiing. The other two main effects are the cooling effect the buoyancy of the water. A good swimmer has a gigantic kick, Sarah Sjostrom once swam 1:09 for 100m just flutter kicking. But yea, the kick of a runner is pretty much useless, it also has to do with the ankles (you want to have different feet/ankles/flexibility for running than would be good for swimming).
That said, when I first measured my HR I was surprised how high it was. I thought I was going easy but it was actually threshold, so a runner with bad technique should have absolutely no problem to get his HR up in the water, unless he is an absolute aerobic monster.
I've found that my heart rate during my swim seemed to be the equivalent of a super super easy run, and I swam for about as long as a super super easy 3 miler would be. Maybe that works?