...just my opinion....

Your opinion is correct. Without gearing, those motors are future landfill. I would use gearing with what ever motor you decide upon.

Let's get some more comments on this one. Someone out there has done this before.

Are 380s too small for a boat that is six feet long, with 2" (50mm is about 2") props? Probably.

I have always hedged my bet on more power. Wind and strong currents can push a boat around. You may want to go with a pair of 12v 550s (such as the MACK 5045) as a minimum.

For gearing, you can look at Harbor Models and Lloyalhanna Dockyard for ideas. They also have some motor / gear combos that will work well. You may even want to call them for a recommendation.

Simple brass gears can be bought from Dumas in a variety of ratios. Electrify makes a gear set for sailplanes that bolts directly to the face of a 550 sized motor with a 2.5:1 ratio.

Sounds like the hull might accommodate a fairly large battery (sealed lead acid) so supplying amps is probably not a constraint. I really believe that 75% of the success of this project will be found in the gears. Once you have a good gear set (3:1 sounds about right), just about any good motor will run efficiently. But 380 is just intuitively too small.