Okay... The Sevastopol is a Premium BB, so there are no special requirements for the BO other than his level (you can use any sailor to drive a PBB as long as the sailor's level is high enough). This only applies to PBBss (maybe other premium ships, but I'm not sure). For the rest of the ships you need to choose ships from the shipline. So, if one of your BO's didn't go down the CV line, he won't be able to drive the Essex.
I don't know if I was very clear, but I hope it helps. :)