I finally got that sucker too (and the destroyer docked beside him) in Yokahama... the trick is, you have to get him to go down relatively quickly. Just because a boat sinks, doesn't mean you get the credit for it. If you've crippled a ship and they sink long afterward, you don't usually get the kill. I guess that's the way it was in the war, too. Many sub commanders knew they'd crippled a ship, but couldn't verify a sinking...
