Mizuho was a seaplane tender, not a submarine. but since they sailed with the invasion force, they never made it to Midway.
I've heard two stories that I'm looking into...one is that some of these seaplane tenders were converted to CVL's...the other is that some, like Chitose, were converted to carry midget subs. Interestingly, another source that escapes me says that Nisshan was carrying torpedo boats to Midway. The midget subs and torpedo boats were for defense of the island after the invasion.
It all sure does get confusing.
Anyway, I'm still looking into the original question on the Mizuho cargo...thanks for the responses so far.
MWA