I also recall reading somewhere that English and Irish fishermen were regularily visiting the Grand Banks off of Nova Scotia and New England prior to 1492 and were certainly aware of the land mass just beyond the fishing grounds to the west. However, they kept that secret to themselves because, well, they're fishermen...
However, what does that matter? Phonecian, Irish, Scandinavian, Chinese exploration fleets, fishermen, may have all visited portions of the Western Hemisphere. They came, they saw, they didn't do much about it.
It was only after Colombus that permanent contact was established between the Western Hemisphere and Europe (and the rest of the world later on).
BTW, Semper Fi!
Africans made successful journeys to the Americas in the 1100-1200s, but they used surface currents to get there so they didn't make it back. Another previous visit that doesn't matter as much as Columbus'.