A blitzball (ブリッツボール, Burittsubōru?) is a spherically-studded ball about thirty centimeters in diameter utilized in a sport unique to Spira in Final Fantasy X and Final Fantasy X-2, where it is the main available form of entertainment. It can also be used as a weapon. Although blitzballs appear resilient against the powerful kicks of blitzball contenders, and the various hides of enemies Wakka and others strike with them, they are light; the material comprising the blitzball lacks the density to sink on its own, and even flies in an upward curve after receiving a Sphere Shot.
Both Wakka and Tidus (for his overdrive) can fight with a blitzball.
You would think that a blitzball wouldn’t make a good weapon. But I imagine a blitzball is the equivalent of our volleyball. A volleyball is about 260-280 grams (a little over half a pound). But trust me, at the speed it travels, it would hurt.
Now consider that the blitzball has to be heavier than a volleyball, as it has to overcome water resistance. Imagine a blitzball hit you in the face. Underwater it might not be so bad, as the water would slow its velocity down. On dry land, it would probably as a bowling ball traveling towards your head at nearly 113 kilometers/hour (that’s 70mph).
Only, a blitzball is light enough for a child to hold.