Since 1986 the International Coastal Cleanup has collected more than 400 million pounds of trash, and its item-by-item counts are the data set behind many plastics policy arguments. You find a cleanup on the map or run your own with the free toolkit, and log everything in the Clean Swell app (available in several languages).
Honest caveat: Ocean Conservancy's site blocked our automated check and the page itself dates from 2022, so confirm this year's date locally (it is normally the third Saturday of September, with events through the month). Logging data is the part that matters most; a clean-up without a count is just tidying.
How to start: oceanconservancy.org/trash-free-seas/international-coastal-cleanup/