IMO which probably isn't worth much, it depends on what you are trying to do with it. Say if you had the 25 and just want to push yourself around with it then yeah I would say you could make do for a while until you get your jet. If your trying to win a poker run, then of course the answer is of course no way.
I have a short shaft (1981) Johnson 7.5 hp that I sometimes clamp on the side of my 22" transom for use on some local lakes with 10hp limits. The water intake for the cooling is well below the water line at all times (even with me, my black lab, and a fat girl in the bow of the boat :mrgreen: ) although the cavitation plate is above the bottom of the hull.
And with that said it pushes my 52" wide at the bottom 14' semi-v starcraft along at a relaxing pace. I wouldn't challenge a guy with spikey bleached hair driving a baja to a race with it, but I don't worry about running my batteries dead on the other side of the lake and having to paddle back, or worse yet ask someone for a tow back to the docks and ramp.
Again, if the water inlet stays below the water with yourself, a big dog, and a fat girl in the bow you should be alright IMO. If you want to be quadruple sure put your case of beer in the bow also as you do the test, then you know that even if the fat girl drinks all the beer and eats you and the dog, she can still go up to the bow without damaging your motor.
