The term "quiet enjoyment" is a legal term that should not be literally applied as you have with respect to the water heater issue that you have written about. In essence the term is applied to a situation where the tenant is to be allowed to use the rental that he or she has in such a manner so to have full benefit of it without interruption.Examples would be where the landlord repeatedly threatens a tenant as to the use of the unit, shuts off the water or electricity and things of that nature. The water heater albeit an inconvenience is not something that amounts to a breach of the covenant of quiet enjoyment under the law. I suggest that you write the landlord a letter about the need to replace the water heater in that what you have written, the build up of sand most likely will cause it to fail shortly.