I see no one answered your question about reduced contacts with GWX. That is correct....GWX does not "give" you near as many contacts as Stock SH3 because it was unrealistic. You will be forced to find where the heavy traffic lanes are and use your hydrophones more often.
When I am in a known traffic lane I will submerge every hour or so and do a hydrophone sweep. You will be surprised how often you hear a merchant that your watch wont see.
Also if you enable the 16km mod your watch will spot many more merchants than with out it.
S! and good luck...welcome to the forums and the addiction of GWX