I play SH3, and although they are both submarine simulations, the feel is quite different, I assure you. Take your pick, U-boots or fleetboats, and we promise not to fight over which one you pick :P
You shouldn't encounter much difficulty in Stock, so if you're completely new, you should go on low realism and destroy everything then move on to using manual aiming and such. If manual aiming is not for you, the super-mods still have the realism options on for you, so it doesn't matter if you're horrible or not. Just have fun playing!
And welcome!