Hi, it seems to be indeed on the side of the boat: coupée.
cnrtl said:
B.−MAR. Ouverture pratiquée dans le bordé d'un bâtiment pour donner accès soit à une passerelle soit à une échelle permettant de monter ou de descendre du navire.
I'm afraid i don't know what l'echelle de coupee is, but i do know a barque is a small boat, so the first bit would be: a spanish boat is going to be moored to...
In english it sounds strange for a boat to moor herself. I also think that berth is for bigger boats and is more commonly used as a noun, although I can't say why. Hope that helps!