Oh, also, when you get into one of those lanes shown on the map, a trick I use that brings results, plot a search pattern on the map. The icon is on the navigation screen, it looks like a zigzag arrow. If you do a medium to large sized search pattern in those shipping lanes, you're bound to run into something. Also, don't be afraid to dive every now and again and listen. Just because you can't see it doesn't mean its not there. Sound carries farther in water
TMO also follows historic events pretty good. If you know of something significant in a time period, such as the Battle of Leyte Gulf, Phillipines invasion, etc. and you're in the area, go check it out. There will probably be some kind of naval force there. The game is pretty good about keeping in line with historic events