Interesting thought, James.
As I understand it, the onset of rigor mortis ranges from 10 minutes to several hours, depending on factors including temperature. Rapid cooling of the body, such as would have occured to victims of the Titanic, tends to inhibit onset. According to what I've read, the first part of the body to be affected are the facial muscles, the maximum stiffness is reached around 12-24 hours post mortem. The condition is caused by the skeletal muscles partially contracting - hydrolysis of ATP in the muscle tissues is the biochemical cause. As the muscles cannot relax, the joints become fixed in place. Facial muscles are affected first, with the rigor then spreading to other parts of the body, with maximum stiffness occuring 12-24 hours after death, lasting over all 1-3 days before tissue decay and leaking of lysosmal intracellular digestive enzymes cause the muscles to relax.
The variable factors in the onset of rigor mortis make it difficult to determine how long after death it will occur - strenuous exercise may induce it earlier (e.g. struggling in the water?), but given the temperature that night I suspect that it would have been, if anything, delayed. I haven't been able to find much on muscle contraction causing a moaning sound - such post mortem noises are usually associated with the escape of gasses during decomposition.
Even if the escape of air can cause a deceased person to 'moan' with the onset of rigor mortis (and I'd like to see the medical literature on that), given the cold that night and the fact that rigor mortis would take time to set in to the point that the chest cavity would be significantly affected I would think the moaning sound would be unlikely to be attributable to this reason.
I'm going to have a hunt around over the weekend in my unfiled papers to find some of the accounts of Hoyt's recovery that haven't been republished yet. Although the inquiry crew accounts don't give the reasons for believing he was alive, there is a remarkable degree of consensus that he was alive when taken into Boat 14, and - although efforts were made to loosen his collar and revive him - he died soon afterwards.
I can't recall off hand any source that mentions a foam at the nose and mouth of Hoyt - perhaps someone could direct me to that? This is one of the prime symptoms of drowning, but is not bloody. The froth is a fine white 'foam cone' created by the mixture of air, water and mucus.
There may be blood present in the middle ear, resulting from conspicuous haemorrhaging possibly caused by barometric pressure.
That Lowe mentions 'bleeding' also suggests that the injuries were inflicted ante mortem, not post mortem or during the drowning process. As he was 'bleeding' when picked up, he would be alive.