I don’t have any personal experience with them, but there are several projects that offer open source plastic recycling and filament extrusion.
For example, Precious Plastics is a good resource (https://preciousplastic.com) They offer free plans for several machines, including a shredding machine to break up chunks of plastic into small flakes and an extrusion machine for turning the small flakes into filament. They also have plans for an injection machine for pressing plastic flakes or pellets into a mold. One of our members (@SeanPo) led a group project to build one here at the hack space last spring (https://talk.vanhack.ca/t/vhs-injection-molder-build-log).
There are a couple of challenges that come to mind:
Plastic Type/Sorting
One challenge to consider will be the type and mixture of plastics in the marine debris. It is likely to be a mixture of a variety of different plastic types with different melting temperatures. This may make extruding it into filament and printing with that filament quite challenging.
3D Printing Speed
Depending on the quantity and size of the trophies, it may take quite a while to print the trophies. (Especially if there are any challenges/ print failures due to inconsistent filament). Need to make sure you allow enough lead time once the filament is ready to do the prints.