Surfrider Foundation's local chapters run regular beach cleanups and log the results; volunteers can also join Ocean Friendly Restaurants outreach and policy campaigns. Surfrider Foundation Europe (surfrider.eu/) runs Ocean Initiatives collections in France, Belgium, Germany and Spain, and Surfrider Foundation Australia (surfrider.org.au/) has 15 branches.
Honest caveat: Surfrider's plastics strength is policy (bag bans, foam bans, the US Break Free From Plastic Pollution Act); treat cleanups as the entry point, then join the campaigns.
How to start: cleanups.surfrider.org/about/beach-cleanups/ or volunteer.surfrider.org/