If you have a coolant leak then the system cannot build up pressure therefore proper cooling cannot take place.

There is a "o" ring between the water pump and block sometimes there will be a leak at this juncture.
The water pumps internal seals also breakdown and the water pump itself will leak. There is a seep hole where the coolant leaks from


Here's an example of a weeping O ring



Replacement water pumps - OEM are Laso and Graf. be wary of Far east copies - e.g. URO.

Not using the proper M.B. coolant or exact coolant reduces the life of the internal cooling system.