It seemed very much dependent on currents, as beaches further south on the windward shore were still pristine.
Maybe there could be some way to help sift things out more quickly? Something the regular beach cleaning / beautification staff could / would do without it adding an entirely new and extenuating task to their work days. My immediate thought was some sort of netting but as you said lots of the microplastics are invisible to the naked eye unless you realllly get down there. That type of net, beyond how expensive it would be having it cover any sort of significant percentage of US / World Beaches, would be pretty much only a nightmare for the majority of sea creatures of all sorts.
The big nets seem good to keep sharks and other nefarious / dangerous sea going animals that could harm humans because the nets are large enough to allow everything BUT those big baddies through. This would be the exact opposite which means literally everything that lives or occasionally travels toward the ocean's coastal areas would be in danger.
What I really want to see is somebody compile, with video, image, audio, and in person interviews with people with first hand accounts / stakes in the ocean's health / stakes in the world's health all of the different types of things like microplastics in the water / on the earth, ice caps melting and the fallout from that, any other evidence about how fast global warming is happening, what types of damage has already happened due to that global warming and obvious water level rise? I think if the average American / Human Earthling saw a video with a large collection of different real life scenarios that are already playing out / have already played out and have a serious, dangerous, and lasting effect on the planet and/or these people's lives / livelihood.
For global warming/climate change- An Inconvenient Truth, which is now probably dated.
I saw a great documentary of the impacts of climate change already being seen in Ladakh firsthand: Jungwa: A Broken Balance. It's pretty depressing to see a village running out of water because of the greed of a society they could never comprehend. Instead they pray for hours, thinking their problems that stem from the environment, are evidence of their own sins. Heartbreaking to see the people that will be impacted the most with the least resources to adapt.
The beaches were nearly pristine, for the number of people there. I only saw the occasional lost sandal on the various beaches, and a few beer cans near bonfire dugouts. I was surprised at the lack of plastic and debris, given reports like yours.
Is there a state or federal agency that combs America's beaches?
This. People don't realize how costly can be to maintain a beach in a good state.
I remember getting early to the beach seeing one of this https://gfycat.com/gifs/detail/BronzeOldfashionedKarakul
Even more expensive is beach nourishment https://en.wikipedia.org/wiki/Beach_nourishment
Cleaning work is ungrateful because people only notice it when you do a bad job.
When we went scuba diving and swimming the consequences of their auto-dredging/sand spraying contraption were horrific, extensive and obvious to all. The organised diving trip to the local reef was to a mainly dead, slime covered reef.
Talking with the locals, who were the maintenance and wait staff, made it plain we'd chosen the "wrong" island for experiencing the Maldives.
It was that holiday and the stupidity we saw that significantly changed our environmental awareness and views.
Also, how do they clean the beaches where automobiles are not allowed? Part of Ocean Shores beach doesn't allow cars. And what about the miles of sand dunes, rocky beaches, jettys and other places cars can't go? How do so few people keep clean so much coastline? Are they training seagulls and deer to pick up litter? It doesn't scale...
An event like the Tōhoku tsunami creates massive amounts of garbage.
But I suspect the locals had read testimony from scientists studying the problem. People really give a shit about pollution in Hawai'i.
It also seems very unlikely that the quantity would be there without ocean currents bringing it in from the big offenders like China.
Hawai'i has a pretty small population of people who are widely very concientious about polluting the ocean, compared to most other people in the world.