Those logs were from old-growth trees, which grew slowly, and thus, had very dense grain, making the wood from them very different – and, it seems, higher quality – from what we get now, from modern, fast-growing farmed trees. In addition, the cold temperatures and low oxygen content at the bottom of Lake Superior helped to preserve the logs. This article (from 1998) indicates that the wood was being used for furniture, woodcrafting, and, yes, musical instruments.