Are Kriegsmarine the submarines? Are U-boats submarines? What's a tonnage? Who's "the rest of them", are they the ships that sank? Sorry I'm not great with history or the navy...
Kriegsmarine was (I think) the name for the German navy. U-boats were essentially submarines. Tonnage was the proportion of weight of the boats sunk. The "rest of them" refers to the rest of the U-boat captains.