Good luck with that. A used fishing float is going to have all sorts of stuff - from barnacles to sand particles - stuck to it, so you'd need a way to separate all that out before you'd have anything that would have a chance of working as a 3D printing filament, which tends to clog up in the extruder even when made from virgin materials. It's also sensitive to moisture absorbtion, so you'd need to cook any water out as well and keep it dry. Even just breaking those things up into bite-sized chunks is going to be a challenge.

While it's a great idea to clean up the beaches which are starting to be covered with plastic debris, 3D printing filament is about the last product that you should be considering making from it. Plastic lumber, for building decks or piers, would be a lot more feasible.