Amazing, huh! It sounds like the landlord doesn't care that water is being wasted. That surprises me at the costs of water and sewer nowadays!
ThanhN, you are not responsible to repair anything in a home that is not yours. In fact, any damage done as a result of you doing work will be charged to you and you don't want that so I would stop worrying about the repair.
Instead, what you should worry about is the temperature and availability of hot water to you/your apartment. Here, I'd recommend that you purchase a cheap thermometer at a local hardware store and check the MAXIMUM temperature of the hot water coming from the shower. If the temperature is 112F to 120F or so then you are fine and the landlord is providing hot water as required by law. If, on the other hand, the water temperature is, like, 100F or less, then you need to inform your landlord that the hot water temperature is not correct and needs to be turned up as required by law. Otherwise, you could actually call the local Board of Health and report the landlord for not providing you with sufficient hot water to clean/sanitize as needed/required, OK?
Good luck here...