Welcome to Subsim.
Which port are you based in?
Also, did you enable the optional Black Sea & Indian Ocean mods by mistake? If these are added, all other campaign traffic is removed from the Atlantic & Atlantic Ports.
If in doubt, use the Merged Campaign as this includes all of the operational areas.
Take a look at the manual as this is all explained.
Of course, if you have harbour traffic, it could be that you are not patrolling in the best areas. It's unlikely that you will see contacts popping up all over the map as you did in Stock SH3, you now need to search them out in high traffic areas while utilising your hydrophone.