Far better to warm the core with hot tea or soup.
I hear that sort of advice a lot, but I'm skeptical. Let's say that I weigh 150 pounds (not since high school, but the math will be easier) and am mostly water. Let's also figure that my temperature has dropped to 95ºF (i.e. probably warm enough to safely ingest fluids) and I can ingest tea or soup at 140ºF (upper limit of reasonable suggestions), resulting in a temperature difference of 45ºF. I hear that a pint's a pound the world around, and maybe I can manage a pint and a half of tea or soup. That's 1.5 pounds, or 1% of what I once weighed. If half of my weight is in my torso hot tea or soup might raise my core temperature by 0.9ºF. Eventually.
Of course it has a beneficial emotional effect, and the localized warming in and near the esophagus and stomach might be useful, but I doubt it really does that much to raise my temperature. And AFAIK the authoritative advice always says to just sip fluids, not quickly drink a pint and a half.
So taking a hot shower seems like a substitute for an electric blanket.
When you're even mildly hypothermic peripheral circulation is reduced. When you're severely hypothermic peripheral circulation is reduced big time, and the blood in your extremities can get chilly. Suddenly having that cold blood show up in your core is not good, and it's dropping your core temperature even more that's the problem, not dropping it again. That's why protocols are to warm the core without dilating the peripheral vessels and letting that cold peripheral blood return to the core quickly. If you're severely hypothermic but not dead yet suddenly reducing your core temperature by another few degrees could change that.
A warm shower probably isn't as bad as wrapping an electric blanket around the entire surface of the body,
but it's not an effective way to concentrate warming on the head and torso and has a good chance of dilating the peripheral blood vessels. Whether the blankets are electric or not, the limbs should go outside them.
Of course very few of us are going to get more than fairly mild hypothermia from diving.