Background: I've been running for 20+ years, and I've been swimming for less than two.
When I run, I can sustain a HR of 165 for a couple of hours, no problem. I mean, I definitely feel like I'm working, but it's a totally doable endurance effort feeling. I swim with the masters three days a week, ~3000yds/day. I often feel totally maxed out in the pool, but when I check my heart rate it's in the low 140s. (FWIW, on a 2+ hour ride I try to keep my HR around 150, more than that and my legs can blow up.)
It makes sense to me that it would be harder to get your heart rate up in the pool because you're non weight bearing and not doing weight transference, but I don't know why my perceived exertion is so much higher? Is this indicative of bad breathing technique? Lack of swimming specific fitness? Other?
Breathe every 15 strokes; your heart rate will definitely go up. ;-}
No, seriously, it's most likely to be a function of the fraction of your maximum cardiac output that the particular activity uses, which is a function of the size of muscle groups employed in the given activity (S, B, or R). The general trend is that running, which uses more/bigger muscles, will require a higher cardiac output, and thus heart rate, to reach threshold. Cycling will use less, and most triathletes will start making more lactate/protons at a lower heart rate, which is why cycling threshold HR tends to be lower. I'm not sure if swimming necessarily follows this trend, but it would certainly make sense if it did.
Unless you're the Hulk.
Hard work beats talent when talent doesn't work hard. http://www.endorphins-for-breakfast.blogspot.com/ http://www.nolimitsendurance.com/