My experience of marine gearboxes is limited to Lister & TMP; in both these cases the input shaft speed is exactly the same as the output shaft speed, so the word gearbox, though true in its strictest sense, would make its function easier to understand if it were referred to as a forward/neutral/reverse box.
The reduction on these boxes, if any, was achieved by using a simple gear reduction drive downstream of the gearbox output shaft.
My impression is that given that the speed range required on canals (dead slow to very slow) is easily handled by the engine rev. variation alone; any difficulty in slow travel or gaining top (!) speed might indicate a propellor compatibility problem?