No, the idea is that each sunk ship respawns after a certain time, like hangars do on airfields.
Its a question of balance, at the moment it is too easy to kill the fleet and too hard to defend it. Manned AA is very effective against jabos and torp planes, much less so against level bombers. Capping a fleet involves circling around for ages waiting for bombers that may never turn up, as this is far more boring than attacking an airfield or furballing few people are prepared to do it. Now if it required all the ships to sunk in a time frame then it would be a lot more worthwhile to CAP the fleet. Even if you miss the first lot of bombers, they or others will have to come back to finish the job. So you are far more likely to see some action and you have a goal to achieve as well, protect the ships until the CV respawns.