Agreed. I actually considered pointing out that in my original comment.

As an American with a fascination in documentaries I have heard the term a lot but most consider it to be very specific to military submarines operated by Germany in the first and second world wars only. I suppose the term Unterseeboot still sees usage outside of the States these days.

FWIW, in America, modern submarines are exclusively called "boats" by people who work on and around them (regardless of size). Definitely not "u-boats." Never heard anyone outside the submarine community say "boat" though and I've had people in the general public try to "correct" me when I said "boat" to refer to a submarine.