It could be normal condensation occuring in the exhaust system. If it is only doing it when you start the car up and lasts less than 10 miles it is normal.
When you shut off the car condensation forms in the exhaust system as it cools. This is why many cars that are used for short trips need exhaust systems replaced more often than normal. The water eats away the system from the inside out. But if it is doing it all the time you have a bigger problems. What I mean if you drive down the interstate at speed for a half hour and it is still doing it, worry.