I was in the southern parts, you were hot and it was already warm. The shower was basically a way to cool off. I would take a couple a day, but they were always quick. and again, water pressure was like 5-10 psi on a good day.
We got an Airbnb in Hanoi at one point, and that had hot showers and good pressure and I took a very long one. Best I had felt in a while.
From what i saw they seemed to be pressure/heat tanks (they were mostly silver or dark and in full sun).
Looked like most places had their own personal water tower.