You can turn off the health bars in the Gameplay Options. Remember though, for the campaign you need to set the Gameplay Options IN BUNKER after starting. The stuff you set from the main menu doesn't apply to campaign games.
Usually spreading damage across the waterline of a ship is the most effective method of sinking on. Four little holes everywhere will be better than one big hold somewhere.
Some ships can be taken out with just one torpedo. Sometimes even one will hit and it looks like it won't sink, but it eventually will. For fast moving ships giving them a nice hole in the bow is good so that they end up plowing forward and drinking the sea in!
