Hypothetical guy suffers a heart attack, falls out of a row boat, and lays face down in the water.
Will he sink right away, or float for a time?
If, and when he sinks, how deep will he go?
Is there a depth that he won’t go beyond, or, will he always go to the bottom, regardless of depth?
Does fresh water or salt water make a difference?
Will he rise to the surface at some point, and barring anything holding him down, when will this take place?
If he’s dead when he hits the water, and has air in his lungs, then he’ll probably float initially. If he’s still breathing, and sucks in a lungful of water, he’ll probably sink right away.
People float better in salt water than fresh water, because salt water is heavier.
I don’t know about your other questions.
A few days ago, that happened to a man at a lake near here. Folks onshore noticed his boat, adrift. When they went out to check, they found his body floating nearby. An autopsy will be done, but he probably had a heart attack and fell off.
Most live people are bouyant in fresh water. (I personally am bouyant, but at a depth of about 4 inches.) As ltfire noted, drowning changes that.
Decomposition inside a corpse will eventually produce enough gas to make it float again, if it has no concrete overshoes.
Ack. I should have credited Invisible Wombat for that. I guess the invisibility fooled me.
Wow, I get to do this!
Okay, upon rereading the piece by Cecil, it does not really address the OP. Er, never mind.
Anyone else think (or hope) that this was about oral sex?
Personally, I like to perform cunnilingus but by the third time my tongue would be very tired.
We’re tricky that way. 
4 inches? I doubt it. Since water doesn’t compress significantly with depth, you’ll either sink all the way to the bottom or float all the way to the top. I suspect that if you lie on your back and relax completely, you’ll have your face above the surface, but maybe not much more. In any other position you are unlikely to notice that little area exposed. You will float higher with a lungful of air, and sink a bit when you exhale.
Eh? You can achieve neutral bouyancy at any point in the water column, surely?
Certainly, most people would float at the surface with a normal lungful of air, but clothing can easily add sufficient weight that your neutral point is a few feet below the surface.
At least, that’s my experience from diving. I could be wrong.
It can’t be. Water is always the same density, no matter how deep you go.
As you go deeper, the pressure will squeeze you and make you slightly more dense.
So, the deeper you go, the less bouyant you get. There are three options for what a body will do in water, if it doesn’t have some kind of active bouyancy adjustment like your BCD when you are diving:
That’s true as a first approximation, but to be comple, we should note…
While water is relatively incompressible for a liquid, it is not absolutely incompressible. That compressibility actually varies nonlinearly with temperature (increasing as the water is heated OR cooled from 46.5 C. Also water’s density varies with temperature, regardless of pressure, and surface water is generally warmer than deeper water (the denser cooler water sinks)
Water has quite a few anomalous and complex behaviors, some of which contribute significantly to common phenomena (e.g. lakes “freezing over” at the surface) which would not occur with most other fluids.