I dont know Hiroshima had a huge Mitsubishi complex there it was a pretty vital hub of industry and I do belive that there was a modest size naval port there as well.
Unless someone mods it in for US subs you cant lay mines and theyed be boring anyway seeing as you lay them and leave in real life there is no way to get credit. Besides minelaying was not done very often by Us navy subs in combat zones.You loose torpedos whe you carry mines.Better to have a sub as a directly offensive weapon.