Well im in mission where i have to patrol right near Tokyo, then i thought why not try getting into the port of Yokosuka, so of i went and so far have spotted the Hiya Carrier and what looks like a battleship not yet identified sitting in port, im at periscope depth within about 8miles of the port inching my way in.
Oh ive gone back to the stock game 1.4 with just ROW mod added (just for a bit of easy pickings

)so i figuring this is why so far its easy getting in, i think if i had TM or RSRDC etc., i would of probably never got anywhere near where i am.
If theres a port nearby your patrol go check it out just dont go to far from your patrol zone.