Heat is a valuable tool. It allows greater stretch response, basically pushing the stress- strain curve to right. Cooler temperatures has the opposite effect. In the absence of heat though I've noticed, in me at least, the penis rapidly cooling to body temperature or even below in about 10 minutes. One reason I usually have a cooldown with stretching. The tissues cool with the fibrils in elongated orientation.
I have tried ice water dunks in past but like you not long enough to really see anything. I might try it with my current protocol in future but right now I don't see the need for it and am progressing better than expected. The gradual cooling might be better anyway since it'll slowly go back to normal temps under load. Our body seems to do better with gradual changes, including adding loads. I now gradually increase tension when using any load, extender, hanging, manuals or lengthmaster work.