Ok I just tried on the mission with the Balao (1945). This mission where you have to sink the Yamato in Kure.
I was at a depth of 45ft, I tried to turn on the radar with the switch and not with the HUD as the button is not active. It didnt work.
I am on 96% realism settings (just the external view possible to check the radar out of the water)
Please check these screenies :
Is it possible to try yourself with this particular mission and settings ?
Thanks !