Powering up your moves in Pokémon Shiny Diamond and Shiny Pearl is useful in any battle, and the Splash Plate can provide it when attached to a water-type pocket monster.
You will find Splash Plate south of Sandgem Town on Route 220. It is located near your starting city in the southwestern area of the Sinnoh region. Before venturing out, make sure you have the hidden Surf move. You will need to go to Celestic Town and get the skill from the elder.
When you arrive in Sandgem Town, go down until you find a beach. Now use Surf to get in the water. Pass the annoying children blocking your path and descend to the left section of the path. You will face a lot of Tentacools and Wingulls in this area, so use some repellent if you have them to ease their annoyance.
Now, keep going down the section until you see two women with pink headbands swimming together. Go south to them and go to the beach. You should see a triangular shaped rock face. Run down the beach until you see the Splash Plate waiting for you.
Splash Plate will boost your Water-Type attacks on a Pokémon on your team. It can also change the type of Arceus in the water.