National Archive is a good start. There are sub-archives in various cities. Atlanta has one and then there is the overall one in the Washington D.C.
The U.S. Navy has museums that may fill in some blanks. Each U.S. Service has an overall office that answers to the Pentagon. The Army had its ships during WWII and the Coast Guard should not be ruled out.
Depending on the country, if they can't answer it, they may refer you to a better source. Of note, to name a few, German U-Boats, the Vichy French lost their fleet to the British air attack, there were landings or several beaches to be traced and the British ran a blockade at Gibraltar. Merchant Marines may have separate files.
Convoy numbers may help as well.
Atlantic, Baltic, Mediterranean
German submarines were referred to as U-boats during the world wars.
The Germans manufactured the u-boats.
They had over 1,000 U-Boats They had over 1,000 U-Boats
they traded by the Mediterranean Sea by boats, and they used the silk road.
German submarines were referred to as U-boats during the world wars.
If there were U-boats in the Black Sea they could have traveled throug the Mediterranean Sea, the Aegean Sea, the Sea of Marmara, the Hellespont, and the Bosphorus into the Black Sea.
To sink ships.
Germany called there submarines U boats
They are actually called 'u-boats', and they were employed by the german military.
Most likely Germany, they had the U-boats (underwater boats)
yes they did use u-boats