In The Two Towers, when Frodo and Sam meet Faramir, he tells them something that happened a few days earlier:
'I sat at night by the waters of Anduin, in the grey dark under the young pale moon, watching the ever-moving stream; and the sad reeds were rustling. ... Then I saw, or it seemed that I saw, a boat floating on the water, glimmering grey, a small boat of a strange fashion with a high prow, and there was none to row or steer it.
'A broken sword was on his knee. I saw many wounds on him. It was Boromir, my brother, dead'
Frodo thinks it's a vision:
'Yet how could such a thing have happened in truth?' asked Frodo. 'For no boat could have been carried over the stony hills from Tol Brandir; ... And yet how could any vessel ride the foam of the great falls and not founder in the boiling pools, though laden with water?'
But Faramir seems to think a boat made by the elves in Lorien might make it:
'You passed through the Hidden Land,' said Faramir, 'but it seems that you little understood its power. If Men have dealings with the Mistress of Magic who dwells in the Golden Wood, then they may look for strange things to follow.'
Is it ever established, perhaps by Word of God (i.e. Tolkien's letters) whether this was a vision or Boromir's actual body?