You could say that the recently deceased don't immediately become a part of the Sea, maybe they first go to the Black Citadel where they might get sold or traded to lesser nobles of the Underworld. (Yep, I read Wraith: The Oblivion in high school. Don't think it ever got played though. Now maybe my D&D game can benefit from that finally.) This is setting up a wheel-and-deal scenario rather than a straight fight, which makes sense since it's probably pretty tough to go toe-to-toe with a lord of the Underworld.
Any way, I've added a picture/link on the right to gather these posts.
Speaking of pictures here are some that inspire the Sead of the Dead: