depending on your liking
I spend few days testing most popular mods like Operation Monsun, Real Fleet Boat, Fall of Rising Sun and what i can suggest for you is Trigger Maru Overhaul.
I really enjoyed playing Pacific Campaing but i have some thoughts about sub hunters. Imho they can see (actually hear) bit too much and im going to ask TMO vets about it in other topic.
If you want to play as german sub commander, check Operation Monsun. OM is highly developed u-boat mod for true Grey Wolfes
Each above-mentioned mod has own environment system with really nice water