If you have a man in the radioshack just tell him to do a continuous sweep and he'll keep going until you dive or stop him. If you go under a wave, though (like 10 meters or so), he'll turn off the radar and leave it off until you tell him to switch it back on. Makes using the radar in high seas a bit of a drag
Also, merchants won't have radar detectors, but assume any escort you come across does. Late war it's pretty much guaranteed they'll know you're there if you use radar.