Welcome to the Pacific theatre.

You'll get at least as many opinions as people who answer.

We do all agree that if you haven't, go ahead and upgrade to version 1.5. It is also known as The U-boat add-on. It's officially called Silent Hunter®: Wolves of the Pacific Uboat add-on.
The mods I use are TMO, RSRDC, MaxOptics, and several smaller mods.
Pay attention to what everyone says, experiment, and find what you like best.