Your maximum heart rate also might make an HR of 136 seem lower or higher. My max HR is 188-192, at least that's as high as I've ever recorded... anyway my average on my last 600 was 131 and I only got it as high as 160 on that one so I don't feel like there's a problem. My resting heart rate last time I measured it overnight was 55... I dunno that there's much one can do besides get the medical tests and try to stay as fit.
In the heat I tend to slow down a bit and try to drink less water, not more. By slowing down I don't need to drink so much and more importantly I don't generate quite as much body heat as I do when it's cooler out. I've read that the most water anyone can absorb in an hour is between 500-800ml so I try never to go over a bottle an hour for myself, since my bottles are ~700ml. I think drinking just enough to satiate thirst is important, drinking beyond that just invites hyponatremia.