There are other skills plumbers need aside from being able to work with water. One of the most common things that a drain leak on is fixing water lines.
Examine the washer and replace it with a new one. Also replace screws in water line repair case they are old and damaged too. The new ones you buy should be durable and of good quality.
Another obvious indicator is if the water you get is close to cold as ice, then your water heating device has already reached its maximum level of service. If it can talk, it might have been shouting at the top of its voice telling you it needs replacement.
The next step in this process is to determine what type of method is available for fixing the problem. You cannot just caulk a crack or fill it in with cement and hope for the best. Unfortunately, there is more work than this because you have to fix the problem from both the inside out and the outside in. This takes a professional in most cases to determine the best method for repair.
The first step in the replacement project is to turn off the water line repair to the toilet and flush any water that may be in the tank. If you forget this step, you will have a giant mess to clean up! You should now disconnect that water line.
When doing the install on a water heater, keep an eye out for unconnected pipes. A commonly missed pipe is the one that leads out from the area for drainage. It is probably a recirculation pipe; that helps your water remain heated without wasting too much water.
The plumber you hire does not normally do heating but will do hot water heating or in floor heating in the house. Some places you can use the water heater for both domestic hot water and for in floor heating if the heating is a secondary heat. Check with your local plumbing codes on this to be clear.