Z-splash Milotic = +3 attack in one turn= shock factor= maybe viable?
The problem with that is that is directly conflicts with Milotic's best ability. Competitive basically asks for an Intimidate attack drop for a Special Attack boost, so that's out for a potential attack set. Cute Charm and Marvel Scale are both defensive abilities, and anything with Z-splash shouldn't be taking damage, focusing more on dishing it out quickly so as not to waste a Z-move and the +3 boost on something that's not going to last. Additionally, Milotic has an attack stat of 60 and a speed stat of 81. While you might have shock factor, you still won't be outspeeding much, or doing much damage. In all honesty, any Z-splash is a shock factor, so you don't have to rely on a pokemon that doesn't even have a good Attack stat to take advantage of.
It's better to use Z-splash on something that can sweep an entire team with the boost or at least take down enough before going down to make using a Z-move a worthwhile tradeoff. The only pokemon that Z-Splash is decent on are Mimikyu (90 attack, 96 speed, Disguise ability makes setup free, ghost/fairy makes 3 immunities for switch-ins, has amazing coverage with just Play Rough and Shadow Claw, has Will-o-wisp and Thunder Wave as options, can sweep by setting up for free and outspeeding/tanking many relevant meta pokemon), Gyarados (125 attack, 81 speed, flying type gives an immunity, Intimidate and Moxie are both good options [Intimidate being a good ability in general, Moxie being total sweeper overkill], has a decent movepool to make use of [EQ, Waterfall, Crunch, etc], can oneshot pretty much anything that's not a tank), and Solgaleo (Better stats than the other two, but is usually banned).