How about asphalt with plastics mixed into the underwater, but not the top layer?

Deferring the problem. Roads get dug up and relaid, roads get abandoned, maybe a few years down the line, we won't be using roads. Mentally insert "Back to the Future 2" clip here. And so a few decades later, we return to the problem.

I forget where I read this but asphalt is perhaps the most recycled plastic out there. Asphalt can be ripped up and then remelted right back into place or moved somewhere else.

Right! They even have road-paving machines that rip up the asphalt; add a bit more tar to replace the tar that seeped away; heat up the mixture and tumble it; lay it back down; and steamroll over it; all in one pass, all in a closed system with no dust escaping. (Ref: https://youtu.be/XKFaC5RYbEM)

Plus, since asphalt’s petroleum-tar binder is liquified during the working process, it would actually act as a solvent perfect for dissolving (certain) microplastics into. The microplastics wouldn’t merely be in mechanical mixture with the asphalt; they wouldn’t exist as distinct entities any more! They’d revert to being a petroleum fraction, in complex with the heavier petroleum fractions of the tar.