I think the upshot is, they were dead when they washed up. True about the palm trees; the stream keeps the temperature stable, even if it's cold. The ag zone here is 5, and yet, along the Lake Michigan shoreline it is zone 6. It always feels significantly colder there though, in the winter.
Irish St Brendan the Navigator is said by some to have traveled to the Americas in the dark ages. There is a well known manuscript from no later than 900AD describing the voyage he took seeking the garden of Eden and how he encountered several unknown islands that some speculate were real and are islands along the northern passage to Iceland, Greenland and possibly beyond. If so, why couldn’t it work the other way, albeit via a more southerly origin in the Americas than St Brendan would have likely ever seen.