Sonar only
 

The thing about sonar, is that it's going to pick up the target a long time before anything else, even when your boat is equipped with radar, so you can begin gathering data on your target long before you pick it up by other means. hydrophone data is less accurate than radar, but it's plenty accurate enough to begin manoeuvring long before the target's in visual range. It's highly labour intensive though. By the time I actually see a target I'll probably have three or four moderately different solutions for course and speed. But it means that when I make visual contact I'm at the point of putting the final touches to a firing solution, rather than just starting out. Sometimes I can have a course and speed roughed out from hydrophone data before I pick them up on radar.

As I say, it's labour intensive and I'm lazy. A lot of the time, once I get radar to play with I forget about the hydrophones, but in the early days, when radar isn't an option, the difference they make is huge...