For a considerable part of the war, subs were forbidden to move into the Sea of Japan. Not sure if that was due to losses or because it was considered too risky. Only at the end stages, when shipping was scarce, did ships frequently visit the Sea of Japan.
And another tip: before you launch your spy into his dingy, ensure you're lying still. The rackett you hear when your sub colides against that rubber dingy....man!
p.s.: same strategy also applies when 'visiting' harbours. Sail into them at night. submerge and lie still in deeper parts until it becomes night again. Airplanes do not fly at night. Destroyers in later years of the war can spot you on radar (yep, their's apparently does work
)