Not sure how it'd work on a moving object like a boat...but if you throw a pointed shadow and mark the point, and then several minutes later see where the mark is, the shadow point will have moved directly to the east.
My guess as to the "sunstone" would be that you'd use two...both in-line and one on each polarization axis, and where the second one went "dark", then they'd be in line with the sun for whatever frequency they were polarizing for.
Apparently my idea would work but is more complicated and less reliable. :)