WIP, and being put into FotRSU as an AI boat, and it will be in Climb Mount Niitaka mod in a couple of months - beta version, not a release. All of the boats in CMN will initially be "stock" compatible. The way to get it now is to use keltos01's
IJN Ko Hyoteki Midget Submarine (21.40 MB) on this site. You might have to do some editing of files and noodling in things. There is also a Jyunsen B mod that has the Jyunsen B, AM, Kaidai and Kaichu boats, at
Jyunsen B mod (312.73 MB), also on this site. Again though, editing of some files might be required.