I have done this when buying "junk" motors to see if they are at all viable.
On some old mercs at least you can shove the cut off end of a garden hose up over the water pickup tube that comes down out of the middle section of the motor. I have started and run them mostly at low RPM and for a short time. I would not rev them up much as they might easily rev off the tach without the load of the lower unit.
Try at your own risk
