“Marine plastic litter can already be controlled through the United Nations Convention on the Law of the Sea (UNCLOS); the London Convention; the MARPOL Convention; the Basel Convention; Customary Law, and many other regional agreements.
Article 194 of UNCLOS, for instance, requires states to ‘prevent, reduce and control pollution of the marine environment from any source.
Measures shall include, inter alia, those designed to minimize to the fullest possible extent… the release of toxic, harmful or noxious substances, especially those which are persistent, from land-based sources… [and] shall include those necessary to protect and preserve rare or fragile ecosystems as well as the habitat of depleted, threatened or endangered species and other forms of marine life.'” http://www.bbc.co.uk/news/science-environment-43115486
“A spokesman for [SEPA] said: ‘Our officers are comfortable that there is no environmental risk to the River Cree as a result of the bottles being released and, following inquiries, understand almost all of the bottles were contained and removed from the river fairly quickly by local residents.’ …
‘It was never my intent to harm the environment. It was more accident, naivety as well as stupidity in the execution. … It was just my intention to send a wee love bottle with a message to someone I had not yet met.'” http://www.bbc.co.uk/news/uk-scotland-south-scotland-40738484
“Scientists working at Bath University have now come up with a method to make microbeads out of cellulose, a starch found in wood and plants.
Tests have been carried which show the beads can be processed to be soft or hard but can also biodegrade into sugar in a short period of time.” http://uk.whales.org/news/2017/06/new-solution-to-plastic-pollution
Has anyone tried using sand? It may be less of a problem for marine life.