I am very happy to see this project! As someone who converted to vegetarianism a few years ago to advocate for animal rights, I am super glad that you are leading the way to create a better world for every creature.
The foundation of your idea itself seems to be strong. People might react to a sign or if they see a group of people picking up trash on the beach, but they are more likely to be inspired if something bigger is done. For example, organizing a 5k to raise funds to help solve littering in your community. Something like this will reach more people and will be more likely to inspire them to make a difference; especially if they see you are a young person. Best of luck with your project!