Yello!~
I'm probably not the best Slosher user on earth, but I do like to use them when I think they'll fill a roll that I haven't seen a lot on my teams. (aka I've only played solo queue and these are basically what I've learned from that)
First of all, Sloshers play really differently from any other weapon type. It's such a weird mix of what other weapons do. Sloshers can be really aggressive when people are distracted, or when they underestimate the sloshers potential. At the same time, they're pretty solid at being supportive and covering turf. My first bit of advice is to figure out ink management, as these things literally murder your ink tank. I usually run at least a thing of Ink Saver main, and the first two slosher kits definitely ask for some Ink Saver Sub. I usually like Swim Speed on the sloshers too, especially on the Deco variant. Make sure you understand how long the Sloshing animation takes, because sometimes hitting that button one too many times can really screw you over. And don't forget about angling your shots. Hitting people over walls is one of the Slosher's selling points for good reason.
For the vanilla Slosher, I usually go ~2 mains of Ink Saver Sub, ~1 or 2 Damage mains, and fill the rest with swim speed or ink saver main. Oh, Bomb Range on this one is fun too, but I typically don't go over 1 main. Obviously there's the Slosh into Burst Bomb technique, so don't ignore that.
For the Slosher Deco, I think I go 1 Damage main, 2-3 swim speed mains, and a bunch of sub saver / main saver. This set is obviously a lot more offensive and is great at walling people out / keeping them away from the rest of your team. I typically save the Kraken as a panic button, but it's probably smarter to use it to push and have your teammates help you out.
I don't use the Soda Slosher a ton, but Splat Bomb + Inkzooka can definitely wreck havoc. I usually use Bomb Range here, just to help you get people around corners or people that are camping higher up. Having a bomb against splash walls is also very useful, so Ink Recovery to charge bombs faster might be a good idea.
Back into general tips, if there's a group of people, I try not to aim at just one of them. it's better to force them to huddle together and get group kills by aiming above them all. Don't have any tips for Splat Zones or Rainmaker (I do think the sloshers are pretty good at hitting whoever's holding the rainmaker, and it'll definitely slow them down with a single slosh), but I would be wary of Tower Control. I usually have trouble hitting people sitting on the actual tower because of how well the little spike-thing blocks shots, but that might just be me.
As for some maps I think the sloshers can do well on, I'd say Port Mackerel, Piranha Pit, Mahi-Mahi, Arrowanna Mall, and Walleye have shown the most success for me. Oh, and Flounder Heights. If you're team can set up, you can easily murder anyone trying to climb up those walls. Slosher in general are really good for inking walls. I think Sloshers are a really good anti-snipe, so it might be a good idea to make that your priority in a match, especially on maps where the sniper perches can't usually be accessed from below.
Last bits of advice, don't overextend yourself unless you're confident in your skills. Whenever I try to take on people alone, it doesn't usually work out too well lol. Again, I do only play alone, but you'd probably do a lot better on a squad with actual communication and teamwork lol.
That's about it, I hope this helped you at least a little!
Oh, and if you haven't already seen it, ThatSrb2DUDE has a pretty impressive flawless match with the Soda Slosher on his youtube channel.
