On a recent trip to Cornwall, I was lucky enough to go to sea, I of course took my rods with me, not fishing rods , dowsing rods, they confirmed what I already knew, that is that the seas are no barrier to the lines.
The lines that I detect are in bands aprox. 200 feet wide and they repeat again on the same bearing aprox every 400 feet.
Once I had fixed myself onto a line it was easy
to steer and keep to that bearing, it was easy to find the same line again when I deliberately lost it.
Although it is easier with two rods to see the bearing, I have little problem just using one rod .
accepting this would be almost impossible in high seas, but I would argue that this would have played an important part in the direction and navigation, couple this with there knowledge of the stars and sun, and the crossing of the oceans was attainable.

yes, there is a school of thought now that the first Norse visitors to North America were following a route known to them through the Irish monks

whether or not the Irish monks were following a route known to them through the Picts, who knows!

Cheers
Andy

As a woman and housekeeper, I must say my first reaction to that story is: I wonder how they made and kept the robes white!