As this has never happened, the risk is is known to be zero.
You only shower in clean water.
What happens with air conditioning is that the water in the tank gets more and more filthy …
Google “legionnaires’ disease” and “water heater” and you’ll get literally hundreds of instances. Where do you get the idea that anything that comes out of a shower head is always pure?
In England and many other countries household water is stored in a tank in the attic. The only safe water for drinking is usually the cold water tap in the kitchen as it is right off the mains. They do this instead of having water towers. Basically every home has its own reservoir of water and having it in the attic makes for an even water pressure.
And yes there have been plenty of cases of things like dead birds found in these water tanks.
How friendly are you and your roommate? Shower together.
I lived in a place for several years with a smallish water heater. When I had a weekend guest, I found it advisable to caution him: “You can take a long shower, or you can take a hot shower. But you can’t take a long hot shower.”
When you’re buying your new water heater, invest in a “low-flow” shower head.
Regarding Legionnaire’s disease : this dilemma only happens if you are using an electric hot water heater. For some reason, gas-fired ones do not have this issue (probably because the bottom of them gets so much hotter during the heating cycle, and this is enough to make it unfavorable)
So keeping it low-temp is just fine if its a gas one.
On-demand heaters are of course the best of both worlds. You set them to a lower temperature, below the scald point, and you don’t have to worry about getting the disease. Also, the on-demand heaters last longer and save about 30% of the energy.
As a minor suggestion, plug the drain when you shower. The warmth of the water will stay in the room instead of just running down the drain.
You don’t say whether it’s a gas or electric model. If it’s electric change your top heating element. That’s the one that goes bad 90% of the time according to my FLI. Worked
for me. Cheaper than a plumber or a new unit.