(Disclaimer, I don't have much competitive experience, only some coaching and scrim practice) I generally agree with the above, if you can hit a Splash-o-matic from where they can't hit you, either behind a splash wall, around cover, or from far away, you'll probably have an easier time. Squeezer is a great weapon because it has the range, accuracy, and kill time to poke Splash from far away, while also having the benefits of shooters in mobility and low downtime that can let it keep up with Splash and pressure it from off angles in ways your traditional backline chargers and splatlings can't. It gets a splash wall to boot, although with Squeezer's range it probably won't have much use for it against a Splash anyways. I'd sing the same praises of H3-D if not for its awful burst fire removing most of those good qualities unless you have really, really good aim.
I don't think sharking is a great tactic, Splash paints well enough that you'll struggle to get within range with something like a Carbon Roller before it shuts off your path to it, especially if they know to take corners wide and paint around them. However if your team has a significant advantage in painting or map control due to the other team being down a player or something, you might find some good opportunities to ninja squid in and splat someone? But generally if we're talking about how to exploit the weaknesses of Splash I don't think sharking is one of them. My best idea for being sneaky would be to have a Dapple Dualies or Splat Dualies quickly roll in from around a corner and use your superior kill time to splat them first? My reasoning is that you'd be able to approach them this way without relying on your own ink for cover or movement. Reading it back it seems a very silly idea though...