During WW II there were several Army Air Corp school/based from Jax South to Key West.
You may have found some ord that was dumped and should have landed in deep water.
I not familiar with that area, but in the South, they used tractors with field rakes to gather seaweed. One could have dug into a stash?
There were several German boats sunk along the coast. Could have had WP for signaling motars?
Here is a map, not all WWII. There are a bunch