From 1600 to 1800 the Dutch East India Company frequently sent its ships into what we now call Fair Isle, the Faeroes, Bailey and the Hebrides, carrying a great deal of gold, to avoid the privateers and foreign navies in the Channel. Storms and unmapped reefs sank several of these ships, and I'm thinking this would have been the main source of gold (and even more silver) in the region - mostly in unminted bars. Interestingly, there would also be a whole load of pepper, cinnamon and other spices from the returning ships, but those were carried loose and I guess wouldn't survive immersion in salt water very well. After about 1790 this supply dried up, which might explain the more recent shortage.

Depth wouldn't be an issue for much of the gold: most of the ships that were lost ran aground, and would have deposited heavy cargoes such as gold close to the wreck event. Moreover, average depth on the continental shelf is about 90m: things get quite hairy between Scotland and Iceland, but there was less call for gold-bearing ships to head all the way out there: that was the realm of the fisheries, and those tended to move bullion around on land but not on the sea: the men who worked them generally came back famously poor.

The alluvial idea's interesting - there are still people looking for gold in Scotland. I didn't know that.