Rent a van. Often this requires drivers with Class 2 licenses. Buses can be chartered. This is the simplest, but most expensive. A local school can probably help you find the most reasonably priced company.
Many public beaches have some access by public transportation.
Ask the beach manager. Walk to the beach if you are close enough. If possible, give a physical description of what the beach and surrounding area looks like. Use the Buddy system. No one will be allowed in the water during the cleanup.