Since the hot water will lower your blood pressure, it is possible under the right medical conditions to pass out and drown, however once you start breathing water, your body is going to kick in the adrenaline and that will usually elevate your blood pressure and allow you to wake.
However, if you were drinking a lot of alcohol, which will lower blood pressure too, you could potentially pass out and not wake up, despite the adrenaline boost. If you are not very healthy to begin with, the lower blood pressure could starve your brain of oxygen and eventually go into a shock or stroke from the heat combined with lack of oxygen. Low enough blood pressure and you wouldnt even be able to reflexively respond if you slipped under the water.
Falling asleep is another matter since the lower blood pressure and the relaxed muscles from the heat are just allowing you to doze off.. Big difference between passing out and falling asleep. The moment you start to take in water instead of air you would react to it and wake up.
I am no doctor or anything, but I have consulted physicians about the blood pressure changes from the heat. Regardless of your health though, I would not soak alone when really tired.
Sorry, but there are several things you sat here that make no physiological sense at all. Yes, water lowers blood pressure, but the accuracy stops there.
