Arguably the weakest and saddest of all the miss-matched fossil Pokemon of gen 8, Arctovish lacks the nifty gimmick it's brothers possess. It isn't as fast or broken in terms of power like Dracovish or Dracozolt, nor does it have the Bolt-Beam STAB of Arctozolt. Instead all it has is boring Water/Ice typing, low speed, and nothing to shore up its strength. Frankly I'd rather use a Walrein, but let's see what we can scrape by with this unfortunate Frankenfish.
Since Arctovish lacks the power of its hodgepodge brothers, let's give it some defensive team utility to set it apart. Set up Hail and then drop Aurora Veil in tangent with Light Clay to give itself and it's teammates some bulk. Hail will also double your speed with Slush Rush hopefully allowing you to outspeed and slug your opponent with a base 170 Fishious Rend which is still powerful even without a boost from Strong Jaws or Hustle. Finally your secondary STAB will be Icicle Crash which will prove useful against the plethora of Dragon types and the Grass types.
Frankly not surprised this "fossil" went extinct, but I tried hard to make it competitively viable at least. Though I guess some creative minds tried harder to justify its extistence:
This is the best I can think of if you want to go on the offensive
Item-Choice Band/Choice Scarf
Crunch/Freeze Dry/Rock Slide/Iron Head/Super fang.
Largely the idea is to mainly hit hard with Fishious Rend and the rest of the moves are coverage. I suppose you could also run hail but that would mean you'd lose the choice items and have to rely on something like life orb.
Arctovish, the last of the Galarian fossils and by far the saddest. Not only is it's typing common, unlike the much rarer Dragon Electric, Water Dragon and Electric Ice typings, but does little for Arctovish offensively and defensively. Resisting only water and Ice isn't good, and the STAB combination is unremarkable as nearly every Water type gets Ice moves and a good chunk of the non Water Ice types get access to Water moves. Being walled by opposing Water types isn't a good footnote either.
Hail Setters -Snow Warning has great distribution so take your pick. Abomasnow, Vanilluxe, Aurorus, Alolan Ninetails all setup that sweet sweet hail that triggers Slush Rush and makes it possible for Arctovish to outspeed targets for Fishious Rend.
Grass types historically pair well with Water types, with both covering for each other's weaknesses. And this is true of Arctovish, though its lack of an actual Fire resistance hurts. Whimsicott probably makes for the best partner, also warding off Fighting types, providing speed control with Tail Wind and Stun Spore, etc.
Checks and Counters
The standard Water type counters work well against Arctovish. Ferrothorn can sit on Arctovish outside of hail and whittle it down. Rotom Wash easily resists both STABs and has super effective STAB of its own, as does Keldeo, but both must be wary of Freeze Dry. Regieleki can't come in safely, but if given a free switch, it naturally outspeeds Arctovish even with Slush Rush and easily OHKOs with STAB.