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A hold item that increases the Base Power of all Water-type moves by 20%. When held by a Pokemon with the Multitype ability, the holder's type will change to Water.
Water Arceus is quite capable on rain teams—Water-type STAB has excellent coverage and is complemented well by Arceus's huge movepool, and it gets even more potent when boosted by the rain. However, Water Arceus's typing overlaps with Kyogre and Manaphy, and often, rain teams could make better use of an Arceus forme that has better defensive synergy with the rest of the team.
However, it is generally useless aside from Arceus, as most other Water-types find the power boost from Splash Plate to be too menial.
Route 13 - Talk to Veteran in house.