Polymer balls that are invisible in water
From left to right, from buttom to above:
Catwoman, Batman, Ras Al Ghul, Hugo Strange, Penguin, Riddler, Deadshot, Black Mask, Joker, Mr. Freeze, Clayface, Scarecrow, Redhood Victor Zsasz Hush, Two Face, Killer Croc, Maxie Zeus, Poison Ivy, Anarky, Great White Shark, Ventriloquist, Bane, Catman, Mad Hatter, Prometheus, Calender man, Solomon Grundy, Firefly, Manbat, Killer Moth. 
