I believe I know where there's a boat cairn in Northern Ireland. Oddly enough no one who can do anything about getting it recognised will ake the idea seriously.
Is this boat-shaped or one with a burnt boat beneath it ?
To make matters worse I believe there is a class of cupmarks - presently being dispelled as 'quarrymarks' - that are specifically boat-shaped. Boats were the vans and wagons of then, probably.