The realism ranges from very simple - eg unlimited fuel, no damage to the boat, etc. - to very difficult, for example, where you have to determine all the values for the torpedo control itself, of course, with all kinds of shades in between.
Really great is the whole thing, once you start, the einzupflegen eg under [...] tons of existing mods. Ranging from small items such as non-Fatigue (the Department does not tire) up to Supermods as Living Silent Hunter, Grey Wolves expansion or SH Commander there are extensions dime a dozen. Some blame the captain's life easier, some the game more realistic, some add tons of new ship types to some change the graphical representation of light and water, and so on and so forth.
On Youtube there are countless videos that represent the look and feel of SH3 much clearer than it is a written description of each can, at a cost of about 5 euros but you just have to strike - not for nothing is considered SH3 also seven years after the first edition still considered the reference for submarine simulations.