No hot water on day one
A landlord must provide a livable home from the start, so no hot water at move-in is the landlord's responsibility to fix right away — and a unit that lacks working hot water may not have been fit to rent.
Hot water is a basic requirement, so it should be working when you move in, not something you discover missing on day one. Report it to your landlord immediately and in writing, and document that it was not working from the start with dated notes or video.
If a landlord rented you a unit knowing it had no hot water, that raises concerns beyond an ordinary repair, since landlords are not supposed to offer a home that fails to meet the basic standard. Keep your move-in paperwork and early communications. If you moved into an apartment with no hot water, a free, confidential review can help you understand your rights.
This is general information about California tenant rights, not legal advice. Every situation is different.