A 8" prop at 700 rpm calculates to about 1 mph. 700 rpm is pretty much standard. If it won't idle slow enough
it has a problem. If on one cylinder it won't idle slow enough. It's not unusual to be on one cylinder and not realize but for the lack of power.
It also may have a idle mixture adjustment that can be adjusted for better idle.
Depending on the boat a 6 Evinrude can make about 12 mph.May have to move weight forward to help it plane.
What year motor?What make and model boat?