She dumped it from her cockpit while I was passing by -- I yelled at her to stop littering, and she said it's "natural" and "biodegradable" therefore not littering.

She was right, and if she was the only person who did that (or if she dumped it in a larger body of water that circulates more with the ocean) it would not cause "significant and long term changes", but given the concentration of boaters in Salt Pond and the lack of water circulation there -- if that was the "norm" it would turn the place into a garbage dump in no time. It's all a matter of degree.