I notice that this figure is getting larger. I saw one earlier today that placed it at 450ft.
Another thing, about early explorers, they weren't necesarily on a mission to find, log and report back. They would probably be well prepared to move to a new land and stay there, or move even farther away. Even if carried to the caribbean accidentally, they had most of what they needed to survive at hand. What I think would add to this debate is a discussion of their cultural tendencies when hunting and fishing. If they did so as families, they would have even less reason to return to their homelands once they found a nice place to set up.
Remember this? It's coral strata from the Pacific Coast off San Francisco.
I would bet that the numbers for the depthe of the Pacific seabed should be adjusted upward to correct for the uplift on the edge of the Pacific Plate (there are credible estimates that the Santa Cruz Mountains would be twice as high as Everest but for the rate of erosion), so perhaps your 450 ft estimate isn't far off.