And because he's Commander James Bond of the Royal Navy. It's traditional.
How did Bond know the pool was full of piranhas? He hadn't been in that room before.
This troper believes that he was only aiming to dispose of Hans long enough so he could be free to terminate the rocket with the keys he'd just seized from Hans (he's got only less than a minute left), not necessarily kill him. Since the "river"'s surface is so far away from the ground, whoever falls into it could have a hard time climbing out of it anyway. If 007 had fallen in (even without the dangerous fish inside) without a Grapple Gun or sth, he would've lost already.