I think what you have is what I call a "hurricane cache" (I heard that term used elsewhere but I like it) Basically what happens is this, a hurricane comes ashore, the winds and storm surge destroy houses and buildings and when the waters recede items are washed out. I'd guess that your collection was on it's way out to sea when whatever container it was in broke open, or got caught up and came apart later. I've heard stories of similar caches being found along the gulf coast, and it only makes sense that they would be found anywhere that hurricanes make landfall.
There are stories that tell of huge amounts of coins being lost to the surge here during Katrina, (both collections, and from businesses) and I'm hopeful I may track one down one of these days.