(this is my first post, so go easy on me!)
With Kyogre holding the title of most used weather inducer in Uber, making a sun team can be hard with all the rain interference. But with a solid sun core backed with a water arceus, a seemingly counterintuitive team can be just the thing to crush opposing rain teams.
Arceus @ Splash Plate
Trait: Multitype
EVs: 252 Hp/ 44 SpD / 212 Spe
Calm Nature (+SpD/-Atk)
- Surf
- Calm Mind
- Recover
- Refresh/ Thunder/ Will-o-wisp
The 212 EVs in Speed lets Arceus outspeed max speed + natures like Palkia and Salamence. Maximum investment in Hp so physical attackers don't hurt Arceus too much, and Surf over Judgement for more PP. This Arceus switches into Water Spout Kyogre easily, and if you sneak a calm mind in it takes Thunder pretty well too. In fact, Arceus is so bulky that a choice specs max spA +nature Kyogre can't even OHKO it before you start cm'ing (although it does leave you basically useless). Refresh is there for toxic, which kills most cm stallers. I don't particularly like thunder, since the only major threat it kills is kyogre, since I like to calm mind stall on kyogres rather than beating it down. Currently testing will-o-wisp as a third option to cripple Extreme killers and the like.
Although Arceus functions well on any team, on a sun team it should only be used as a late-game sweeper (when Groudon is gone and it's raining). I found that in most of my games I either used it as nothing more than a bulky pivot, or as the game-changing sweeper; nothing in between. Always watch out for physical attackers and ferrothorns (protect+leech seed+4x resistance is a pain), but once the opponent is down to solely special attackers, Water arceus will eat the opponent's team alive.
I'm still in the process of testing and may change my opinion over time, but as of now Water Arceus is a great use of a slot on sun teams.
With Kyogre holding the title of most used weather inducer in Uber, making a sun team can be hard with all the rain interference. But with a solid sun core backed with a water arceus, a seemingly counterintuitive team can be just the thing to crush opposing rain teams.

Arceus @ Splash Plate
Trait: Multitype
EVs: 252 Hp/ 44 SpD / 212 Spe
Calm Nature (+SpD/-Atk)
- Surf
- Calm Mind
- Recover
- Refresh/ Thunder/ Will-o-wisp
The 212 EVs in Speed lets Arceus outspeed max speed + natures like Palkia and Salamence. Maximum investment in Hp so physical attackers don't hurt Arceus too much, and Surf over Judgement for more PP. This Arceus switches into Water Spout Kyogre easily, and if you sneak a calm mind in it takes Thunder pretty well too. In fact, Arceus is so bulky that a choice specs max spA +nature Kyogre can't even OHKO it before you start cm'ing (although it does leave you basically useless). Refresh is there for toxic, which kills most cm stallers. I don't particularly like thunder, since the only major threat it kills is kyogre, since I like to calm mind stall on kyogres rather than beating it down. Currently testing will-o-wisp as a third option to cripple Extreme killers and the like.
Although Arceus functions well on any team, on a sun team it should only be used as a late-game sweeper (when Groudon is gone and it's raining). I found that in most of my games I either used it as nothing more than a bulky pivot, or as the game-changing sweeper; nothing in between. Always watch out for physical attackers and ferrothorns (protect+leech seed+4x resistance is a pain), but once the opponent is down to solely special attackers, Water arceus will eat the opponent's team alive.
I'm still in the process of testing and may change my opinion over time, but as of now Water Arceus is a great use of a slot on sun teams.