I was a bit surprised that we got this current sub cancer after all and with the few hybrids already around WG didnt pick the easy low hanging fruit of Japanese float plane subs. Besides I-400s there were like 35-36 others in 2-ish classes and they saw action through the war from December 1941. They could play like cumbersome torp DDs that can hide underwater for some time and with a plane they could use to scout for the team.