Sorry to insist on the matter, hopes that for a good cause.
To make a mod for visual detection issue is probably just a matter (but I don´t have any thin idea on how to make it or the probably hard time to make it) to find the sensor visual max distance memory location (as you did for asdic) and if the enviroment is day left it as is, if is dusk cut it to 2/3, if night just cut it to a 1/2 or 1/3. Perhaps could be also need to make it to Ai visual sensors ... (but Ai visual sensors at night is already more responsible on sim.cfg/Ai_sensors.dat than the uboat crew ones).
Excuse me again to stay on this matter.
Cheers mate!