I play Japan alot, and well after the transports our up and running and asia is under control, I like to build BB’s and Destroyers, just for the psyce value.
The attacker decides whether to retreat/submerge before the defender decides whether to submerge, so your friend could retreat before you could submerge.
National Advantages. NA’s. They are optional rules in the OOB rule book that give a different ability to certain units or make an exception to certain rules depending on the country. I haven’t really played with them but it is said that they add some variety to the game.