Playing Online Question(s)

I have only played this game in single player mode. Am I missing anything by NOT playing it online? Is online play similar in par to the single player game and share a campaign with others? So I could start at Wilhemshaven and someone else at Kiel in the same year/date? Able to spot them on my map and cruise with them in a wolfpack situation and attack the same convoy(s) as them? Able to send messages to other U-boat players via radio room? Or is it possible to have several players on the same U-boat?