Below are excerpts, with my comments, from the declassified summary report of all interrogations of all crew members of U-128. You can find the complete document here.
The German U-Bootwaffe and many surviving veterans vigorously maintained the myth that all U-Boat men were volunteers. This has been dis-proven many times in memoirs and in reports such as this. The other key fact in the above paragraph is the large number of experienced Petty Officers, 13 of whom were regular German Navy sailors who had enlisted long before the war. Men such as these would have proved vital in keeping this U-Boat and any U-Boat operating.
They certainly were. German U-Boat losses had become so catastrophic in May of 1943 that the submarines were withdrawn from the North Atlantic and sent to less dangerous areas such as the waters off Brazil where U-128 was sunk. The US maintained substantial anti-submarine forces of ships and aircraft in Brazil which declared war on Germany and Italy on August 22nd, 1942.