I'm not clear on where you put "prize ships" once they are captured. The rules state that they count for double points and they are place on your victory point deck. But, it also states that these points are not awarded until the end of the round. This hints, that the "prize ship" could be retaken or sunk. Is this correct???
WHERE in the playing area is the prize ship supposed to go when you capture it???
From the rules:-
3.2.5 Prize Ships
An Intercept! of a Merchantman may result in its capture. Captured Merchantmen are referred to as Prize Ships and are added to a player's back row along with their Warships & Raiders. To capture a Merchantman, a boarding party and/or non-combatant card must be played as part of the Intercept!.
Does that help?