There are loads of places to visit so I’ve just selected a few which I like the best. And please avoid disappointment and don’t send me more links, no matter how useful you think they are. Lots of people do but I simply don’t have time to deal with them. Sorry!

Trash Free Seas: Campaign from Ocean Conservancy. Harnessing the Power of People to Fight Ocean Trash, September 2020. This is a really great website with loads of resources and ideas.
5 Gyres: Here’s a great site which deals with the problems but then has the solutions. You can certainly get active through this site!. If you’re in school, there’s plenty of stuff for you in their education section such as the 5 Gyres Trash Academy
Marine Debris (NOAA): One of the best websites for info on the problems of plastic and the solutions

Where is all the plastic?: Every year, the amount of plastic people make increases and most of this soon becomes waste. But scientists had been puzzled as to where all the plastic waste was going. Much more plastic was being made than could be accounted for by waste plastic on the world’s seashores and the ocean gyres. This critically important work shows that ALL (!) the planet’s deep sea sediments on which myriads of tiny creatures feed, are contaminated by microplastics. Royal Society Open Science, December 2014
Surfers Against Sewage: An environmental charity protecting UK waves, oceans and beaches. SAS shows ways you can get active in plastic clean-ups

Plastic Pollution in the World’s Oceans: More than 5 Trillion Plastic Pieces Weighing over 250,000 Tons Afloat at Sea. Published December 10, 2014 by PLOS, this is definitely for the serious researcher

Some videos and a slideshow