If it's plain water (no sweet smell of antifreeze and your coolant isn't
disappearing), it's normal. I did the chemistry once, and the saw is true:
burning a gallon of gasoline produces almost exactly a gallon of water. When
the engine and exhaust system are cold the water vapor quickly condenses. It
either collects somewhere in the exhaust system to corrode things or is
spewed out the back. As the system warms up it evaporates.

Mike