Well, in the rain the power of Fire moves is lowered by 50%, same goes for Water Sport. So let's say the oppoenent has a 100 base power Fire move, and is not getting STAB. I'm pretty sure such a move does not exist be we're pretending it does for the sake of simplicity. In the rain its power is halved to 50 power, and then, 25% of 50 is 25, is with Water Sport and in effect its power would then be a measly 25. With that in mind, Water Sport should probably never be used competetively.