I can offer a BIG thumbs-up for SH5 with TWOS; it's very immersive, you have the feel of actually commanding your boat, not just clicking the screen. Your Bosun - if he's well trained - takes good care of the men, your cook can lift their spirits. There are rivalries and animosities amongst the crew. Tragedies as well...this is wartime.
The vessel upgrades are excellent and really give a feel for the developing war. The opposition ranges from lacklustre - as in my game at the moment, off the coast of America - to bloody dangerous: the Brits around their soggy little island.
I loved SH3 and the mods are superb, but for right now I'm strongly suggesting SH5 with TWOS.
Cheers!