Welcome to Subsim!
I only play stock SH4 v.1.5 but I've found the Luzon Strait to be an excellent hunting ground, particularly later in the war. The Formosa Strait (between Formosa and mainland China) is also excellent, but a little too shallow in most places for my liking!
Also, in late 1942-early 1943, the area of the Solomon Islands known as "The Slot" is also not too shabby, particularly near Bougainville, but you might run into lots of small craft such as fishing boats.
Hope that helps! Happy hunting!
