Even with a speed controller I think it's still too fast.
For that size of model and prop I would say a motor with a free running RPM of about 6-7kRPM is more appropriate , and it will be quick at that RPM. Bear in mind an Engel 212 manages about 7 mph submerged with a motor turning at about 5kRPM.
A good little geared motor would be the little GWS IPS packages. these were used a lot in slow flyers a few years ago. Long since surpassed with brushless motors, they still offer a very good power to weight ratio. I have two here, an 'A' unit with 5.9:1 ratio, and 'B' unit with a lower 7:1 ratio. I would say the 'A' unit would be just right, and you'll get lots running time, as they're quite efficient little units. Dead easy to mount too- a rectangular barb slides into the back of the unit.
Anyway, have a word with your WTC builder, and see what he thinks.