Here is a picture of the plug and its socket

One of our Model Boat Website members helped develop this kit. Tompallister. He posted a video of his model in 2013 in our Video Gallery. http://model-boats.com/videogallery/4
The sleeving suggested by Haverlock is fairly essential and I am surprised none is provided with the motor.
Without seeing the plug it is difficult to give detailed advice. Do the plans not indicate how to attach the plug? is it possible the wires just push into the plug? Some electrical connectors (especially of the minature variety) allow a wire to be pushed in but will not allow its withdrawl.

If you can post a pic it would enable more specific help

They probably are insulated but if your worried get some sleeving

http://www.maplin.co.uk/search?text=sleeving&x=0&y=0

slip it over each wire prior to soldering on the plug.

if you have a multi-meter you can check I suspect the wire has an insulated coating on it as its probably the same wire that was used to wind the internal coils.

its commonly known as "enamelled copper wire"

https://www.wires.co.uk/acatalog/cu_enam.html

hope that helps