I assume the author knows why they're called U-boats, but the fact is that modern English speakers don't call them U-boats unless they mean very specifically German World War II submarines and maybe not then.
I would use "U-Boat" but only in the same way I'd use "Boomer", to indicate a specific type of vessel in a particular context, if I meant a submarine, such as in a simple drawing, I'd say "submarine" and so would other English speakers I know.
And probably also German WW I submarines which were a factor drawing the US into WW I but might be less discussed as the war had less impact on the US and was longer ago
I would use "U-Boat" but only in the same way I'd use "Boomer", to indicate a specific type of vessel in a particular context, if I meant a submarine, such as in a simple drawing, I'd say "submarine" and so would other English speakers I know.