Water quality should be good, most of the water is right out of the bottom of Chatfield. You may want to wait a little while after a major rain so the urban runoff component of the flow decreases. Again, most of the water is dam release, so flow is not runoff dependent, per se.
So far as safety, I'm not aware of any major hazards for a strong boater. At this level, play is good, some of the waves/holes at Union (and probably above) are powerful and could seriously pummel a novice boater. For example, a swimmer may be caught in a room-of-doom style eddy with eddy fences too strong to swim through.
The lines through each boat chute are less clear at high water. When in doubt, aim for between the two rocks, which mark the gate. Failure to do so could send you into rip-rap, strainers and other nasties.