A subjective interpretation of what is high, low, or correct RPM is somewhat irrelevant here. The model prop rpm will never match the 1 to 1 scale prototype, the pitch and surface area of course are radically different in size, and the density of the water (allowing for that we are discussing here fresh water instead of salt water) are a relative constant. Basically written, you can scale down everything but the water.
Perhaps a better measure would be to find out (with your 3 to 1 gear [or belt]) ratio what amperage does your Graupner motor draw? A 3 to 1 ratio sounds correct here. Take a amp meter and test what the motor draws while the propellor is immersed in water. The stern end in a swimming pool or bath tub. Considering the size of a 1/96 Ohio, then you better consider a pool!
The less draw, the cooler this motor works (the motor will get warm anyway) and the longer the running time will be from the same given battery charge. A large motor, like 1 to 2 amps might be acceptable for a 550 Graupner, lower the better. Careful alignment relationship of the motor, shaft, water tight seals, and bearings can effect this. It varies much, if it was a beautiful Maxon motor, I would be concerned about 1 to 2 amps being maybe too high. So it depends. Graupner motors are fine for the price, though I am sure everyone has their favorites.
Build her good and straight forward water tight first, then refine her later as you grow in experience as to what is important and what is not. Try not to succumb to analysis paralysis. It is very easy to do, I have! Hope this helps.