A situation like your drowning example is also irony; sometimes to be explicitly this is called "situational irony" as opposed to "verbal irony."
I apologize for being pedantic; mostly I just wanted a justified link to Silva Rhetoricae: http://rhetoric.byu.edu/figures/I/irony.htm (but see also http://en.wikipedia.org/wiki/Irony).
I don't even care if that's true of all of us Americans, it's true of enough. (The idea that Americans don't get "sarcasm" is very foreign to me; oh, I certainly know people who don't get it but we seem a fairly sarcastic culture to me.)
On the other hand, one can reasonably believe that every dictionary is, by its nature as a reference book, primarily prescriptive (and proscriptive) and therefore cannot be reliably descriptive. If so, then it seems we do have to go only with personal observations.
In terms of personal observations, I agree with you that many Americans I know would use "sarcasm" in this case and not use "irony." I think I (along with many other Americans, maybe fewer than the other group?) would use "irony" and probably not use "sarcasm" because it doesn't have that particular, hard-to-miss sarcastic tone of voice.