No, you acutally have a chance to enter the area if you observe him and are patient enough to wait for him to leave.
http://eoc.eri.u-tokyo.ac.jp/harvest...UNGO-SUIDO.jpg
He starts at around 34', 131', and then patrols counter clockwise from there. He goes through the western portion of the bay, then into the straight. He then turns at around 33', 132.5', and then goes back up through the straight and then patrols the eastern part of the bay and then returns back to 34', 131', where he knocks off for a couple of days.