Spray paint water guns with flat black paint specific for plastic/patio furniture, then kinda keep the camera not fixated on them.
One consideration is that viewers can still note weight, or absence of, in the way the actors manipulate the light plastic, so you still gotta fill 'em with water.
If you're going for a sci-fi/military exotica look then spray paint a nerf dart gun.
And spray in the tiny nooks, crevices, and cracks on your first pass.
Leave the exposed handling surfaces for last.
There are youtubes for this.
https://www.youtube.com/watch?v=TUjoe03VCw
https://www.youtube.com/watch?v=hE-ZmwATS8E