Simple solution: learn how others play weapons you struggle with, and learn how to cover up those issues whilst exploiting weaknesses in their playstyle. The Splash is not an OP weapon, it isn't even top tier for quite a few reasons. As Elecmaw has stated there are no OP weapons anymore, we've come a long way since May 30th 2015, we've had new weapons, new maps, new gear, new modes, and constant balance changes for at least 14 months. I don't know what mode you're playing or what rank you are, but I rarely see Splash's, and when I do it's usually the Neo because it's the superior version. The Splash has less range than the N-ZAP and Splattershot, a slower strafe and doesn't really have any viable points against most weapons. All you need to do is learn how to play against weapons rather than cry how they're OP and need nerfing. Nintendo and the Splatoon devs are done with Splatoon, any patches now are just bug fixes, now that Colour Splash is out there's only one game left for Wii U from Ninty and it's BOTW. All focus is shifting to the NX so no changes will be done.
TL;DR: Learn how to beat other weapons, no more balance patches are coming, Nintendo is finished with this game and all focus is on NX