If your water heater smells like sewage, it’s possible that you have the same issue that commonly causes a rotten egg smell, as described in this article. This will usually be due to a buildup of unpleasant bacteria in your heater, which can often be solved by replacing the heater’s anode rods or flushing the system with hydrogen peroxide. After trying either fix, leave your taps running for a while to clear out any remaining bacteria.