I think mods are pretty essential! The basic game has far too many realism problems and outright glitches. I wouldn't recommend it without mods.
There are many flavours, but the most common ones are:
If you're looking for Pacific action, RFB (Real Fleet Boat) or TMO (Trigger Maru Overhauled) are two basic mods you can choose from. They can also optionally be customized with RSRDC (Run Silent, Run Deep Campaign), which is a very thoroughly-researched historical campaign layer for the game.
If you're looking for U-boats, whether in the Indian Ocean or the traditional theaters (Atlantic, Med), OM (Operation Monsun) is the most obvious choice. It also goes very well with OMEGU (Operation Monsun Environment and Graphics Upgrade).
There are of course many other options, but these basic choices are excellent starting points.
Happy sailing