The Long Island Sound Study (LISS) website has a great Volunteer Opportunities page. It lists volunteer opportunities relating to the conservation of Long Island Sound with over 30 different nonprofits and government agencies in CT, and even more in NY state.
The biggest annual event, International Coastal Cleanup, is in September. Save Our Sound, a program of the CT Fund for the Environment, organizes the cleanup in CT. Here is a link to the Calendar for the cleanup (but events have not been put up yet). Also check out the Don’t Trash LI Sound’s media center page.
The LISS website also has a What You Can Do page that links to five different sections of the website. The sections are In Your Home, Around Your Backyard, In and Around Your School, In and Around the Sound, and Curbing Polluted Storm Water Runoff.