I think you might be misreading - what is more likely is that it says "900,000 tons of shipping destroyed". What this refers to is the total amount of tonnage sunk. This is usually measured either by deadweight tonnage (ie, the physical weight of the ship sunk), or gross register tonnage (GRT), which is a measure of a ship's internal volume (a register ton is equal to 100 cubic feet or 2.83 meters squared). GRT is a useful measurement for navies in this context, because it harmonises the discrepancies in size of ships sunk. For instance, if U-100 sinks 10 small ships, but U-101 sinks 5 larger ones, rather than say "oh, we've sunk 15 ships", the Germans would check the records and registration of the ships they thought they had sunk, and thus gain an accurate figure for the total amount of "ship" sunk. In history books, the way it is usually expressed is: "In her career, U-103 sunk 30 ships with a combined gross registered tonnage of 100,000 tons" (these U-boat names and numbers, btw, are just illustrative).

As for the cargo, it could have been virtually anything - Britain found itself having to import various critical war materials, not least of which was food.

As for the sailors - if they were lucky, the convoy escorts and other ships would recover them swiftly, but if they were not, then they would have to take their chances in open boats in the North Atlantic and pray for rescue. If they weren't, then they would die.

Hopefully someone more qualified will be along, but this answer should go someway to answering your questions.