Right now many of the loons wintering in the ocean have lost the ability to fly. They're simply molting, growing a whole new set of flight feathers before their migration back to their breeding grounds.
Wing feathers are so light and buoyant that they float.
For loons to dive and chase fish underwater without popping back up like a cork, their wings are as small as possible. But feathers get worn out, frayed and weakened, and so loons grow new ones every year. These new feathers will carry them hundreds, or even thousands, of miles to their breeding grounds in a month or two, and then carry them back to the ocean again next fall. It takes a lot of energy and body resources to grow feathers. Loons grow a lot of large, stiff feathers all at once.