To add to Mr Midi's answer...here is the "Why" it is that way explanation...
Without a map room, their is no trigger - or object mechanism available to the running program to effect the transfer of ownership of the boat to another country...
The program is hardwired to look at the "owning base country" of the CV, and then compare it to the current listed country of the CV when it enters the Re-Spawn event..
If the two differ...and to facilitate a transfer of ownership...a message is sent emulating a base capture to the attached master maproom...with the new owners (country) as an argument of the function call...
In essence the map room is destroyed behind the scenes...just as if 10 troops of the country that took the master base also ran into the cv maproom...of course with the arena contingent time delay...
In practical terms: It is as if the 10 troops that took the master base...were airlifted to CV maproom by magic...and told to stand by...when the spawn timer kicks in...and IF and ONLY IF the base is still owned by the new owners...those troops are then told to take the CV maproom...
That maproom function call then calls the boat and tells it that it is under new management...then releases control back to the initial calling respawn event...
The boat then appears on the map under new ownership...
Without that maproom...none of this would be possible...and avoids the inevitable dilema and problems of placing maprooms on the boats themselves...
Oneway