I don't think I see the distinction between your examples. There's no more evidence that the sailor has drowned (or is even dead) in your second example than there is to assign nationality in your first. Both conclusions are not unreasonable however, given their circumstances.
In the online dictionaries I've checked, lack of proof is usually specifically mentioned only when
presume is used in the legal sense. What I can gather from various online discussions of this is that
presume should be used in the absence of proof, with the expectation that proof will be forthcoming, while
assume suggests no intention of seeking more proof. I also gather that a lot of people have trouble seeing the difference between the two words (re the taking for granted definition).
One tidbit I ran across is that
assume is used 10 times more than
presume in spoken and written English (there was no attribution, so can't say if it's true
).
Do you think people in the UK use
presume more often, or differently, than Americans do?