Underwater Clean up campaigns and Project Aware dives against debris are fun and world wide popular.
http://www.projectaware.org/project/dive-against-debris
We've(San Pedro Dive Shops) had numerous such events over the last 15 years at no charge to the participants.
The results showed us how lucky we are that we have very little underwater debris. PROBUS is correct our large problem is beach trash. Garbage that the world dumps into the oceans floats ashore and floats ashore on Ambergris Caye. Our Island got its name from how the Trade Winds and prevailing currents bring what floats in the Caribbean to our beaches.
http://scubalessonsbelize.blogspot.com/2008/12/project-aware-eco.html
Involvement in Beach Clean up can be very gratifying for divers and reward for involvement in the past has been fun dives and snorkeling trips fro participants.
http://scubalessonsbelize.blogspot.com/2009/10/aware-kids_17.html
http://scubalessonsbelize.blogspot.com/2007/08/help-monitor-ambergris-cayes-reef.html
Its easy to organize the community is very aware of the problem and events are often.

White Sands Dive Shop
https://whitesandsdiveshop.com/