Can a landlord stop me from having hot water in the day time

No. Hot water is a given amenity for an apartment. The landlord could ask that you conserve. They could even charge you for excessive use depending on the terms of your lease. But they can't shut it off. That would create an unliveable condition.

I agree with Scott but want to add that the landlord can have the hot water shut off for periods if they are doing repairs or replacements to the plumbing system. However if they are taking an unreasonably long time to do this then you should be entitled to some compensation such as a discount on the rent.