If I've got the math right, at level 18 with no attack speed bonuses other than base, per level, and Pow-Pow factored in, and without accounting for the 10% bonus damage from rockets, you must hit ~2.1 enemy units per rocket to have higher dps with Fishbones than Pow-Pow.
With those same conditions, plus Jinx's usual purchases of Berserker's Greaves and Statikk Shiv, you must hit ~1.714 units per rocket to beat Pow-Pow's dps.
Realistically, you're not likely to get these exact scenarios. In any real game you'll also have some attack speed from masteries, maybe runes as well, and you'll be doing 110% damage with rockets rather than the 100% I worked with. Also, don't forget the bonus range with rockets; that's not to be taken lightly.
TL;DR:
Just hit more than one enemy champion per rocket and it's worth it.