There was only one
submerged v submerged sinking during the war as mentioned above
21 other uboats were sunk by allied subs
http://www.uboat.net/fates/sub-sunk.htm
10 Allied subs were sunk by uboats
4 Russian,3 British, 2 French and 1 American
http://www.uboat.net/allies/merchants/warships.html