55Gallon drums are a good idea. I have done this with small gas engines as well, at wide open throttle for short bursts. My recommendation is to build some kind of test stand out of wood and hook up your motor to it. It is possible to substitute the drum for a large size garbage can, plastic seems to work best. Fill it with your typical garden hose and you should be fine. Alternatively, set up your test rig on the side of an INGROUND pool. Safety warning, please make sure to keep anything clear from the propeller, as it will hurt.
Notice I also said inground pool, above ground types don't have that much lateral strength.
I'm not sure how you would measure RPM below water, perhaps optically, using a video camera. I would try to paint one of the blades white and count how many times the white blade appears over a given period of time.
Although you have not stated your end goal with rpm measurements, I suspect you intend to determine the rpm of the propeller in order to determine your boat's speed. Please remember that this is NOT as straightforward as it seems. Propellers (screws) don't behave exactly like wheels. You cannot determine vehicle speed as a function of rpm quite so simply. There are fairly complicated fluid dynamics at play. Boat shape, displacement and environmental factors (waves, current, wind, etc.) also play a role.
Having said that, it may turn out to be a worthwhile approximation for speed to measure rpm. I'm not trying to discourage, just inform!