Any fossil fuel releases carbon dioxide and water, as a consequence of combustion. In most of the cases, water evacuated through the muffler is nothing to be worried about, as it simply results from fuel burning. Gasoline and diesel are chemical compounds composed mainly of hydrogen and carbon (hydrocarbons). The combustion process (the combination of hydrocarbons with oxygen) results mainly in carbon dioxide (CO2) and water (H2O) in the form of vapors.
When starting a cold engine, the exhaust route is cold and the vapors condense, being evacuated as water drops. As the engine and the muffler heat, the vapors will no longer condense, the leak will stop and the muffler will dry up. This leaking may become visible also when it is cold outside.
In the case of cars that use gasoline instead of diesel, the amount of water in the combustion process is higher, due to the higher concentration of hydrogen in the molecules.
However, the muffler may also leak because of a problem if that is the case, ask around for muffler shops near me to find the most convenient muffler shop in your area to have it checked out. Make sure to check that the level of the coolant fluid does not drop significantly, otherwise the problem may be with the burner head gasket. In this case, the cooling water is also evacuated through the muffler.