Question

Splash plate
Wave Incense
or
Mystic Water?

Comments

Do you mean Wave Incense?

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I thought there was also a wave plate, but I guess i misread it. So sure wave incense.

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Well, in that case the answer is same. There, I edited.

Answer 1

Mystic Water: 20%
Splash Plate: 20%

Use either. They won't do anything different. You also wouldn't want to use the Wave Incense as it only raises the power of Water Type Moves by 10%.

Answer 2

They do all the same at 20%.