I cannot guide you directly to a motor... What I CAN do is first of all point out that there are motors that are wound to spin 2500~3000 rpm and that would save you the hassle of a reduction drive, as well as the possible gear-noise. For 12V you would be looking for motors with a kV rating of about 250, give or take.

But if I may be so frank: 2500~3000 rpm on a 4"fourbladed prop, you're talking about very roughly 250~400 Watt (power calculations on ships propellers are notoriously tricky), and that is an awful lot of power, even for a 56" boat. You're easily going to pull a rowboat with two adults in it around with decent authority. 250 Watt with certainty is more than a single oarsman in a rowboat can sustain for reasonable duration, so your boat will win in a tug-of-war, that kind of authority... Careful estimation it's something like 10 or 15 lbs bollardpull, probably more,

Anyway, whichever motor you choose (high RPM with gearing or direct drive) you'regoing to take into account that for example on 12V it is going to draw at least 20 Amps, probably significantly more, for a geared drive you would be looking for about kV1000 and rated for miminmum 20 amp, for direct drive kV250 and 20 amps (I think the 20 Amps is the bare minimum you should look for).

Just for frame of reference: A Sullivan starter as used for glow engines (planes) is about kV250 but only rated for approx 10 Amp... So that is a fairly big motor you would be looking for.

I myself am currently working on a 44" boat with a 4" prop, and I am targetting 1500 RPM, which should be about 80~100 Watt, and theoretically result in at least 6 lbs bollardpull. In-water tests in the bathtub it was quite a handful to hold in place by hand...