I personally don't think this is necessary. The First Edition game had this problem that once you lost your sea area, it was extremely difficult to recover. So they added ports to the first expansion, which gives you an out once you lose your sea.

Sea battles are more unforgiving than land battles, but the defender usually has a big advantage since there is no siege tower equivalent.

The example you present ... yeah, there is no way Baratheon can get his sea back unless you vacate Shipbreaker bay.
But there were several steps to get to that "irreversible" situation. You had to first invade his main sea, then take Dragonstone, then take Blackwater bay. There are several steps there where he was unable to recover. If it would be impossible for him to recover from just one move, I would agree with you.

Turn 7 and he's lost his seas. Adding the option to CP wouldn't help bara much. There's only a few turns left so the time to recover would be too short anyway. Bara is in a situation where he can't win anymore, your option won't change that.

Also those CP's to muster ships would just get raided by the enemy ships or nearby land forces anyway.

Part of the game is protecting your seas. A good player will usually succeed in doing this without strange options.