From the Super Mario Wiki
The Mushroom Pond is a location in the Mushroom Kingdom in Super Mario Odyssey. It is a lake located left of the Odyssey with a waterfall that pours water into the lake and is directly in front of the kingdom's Koopa Freerunning Ranking Board. Inside the lake is a blue Dorrie with three regional coins on the its back, and a small nook with three more regional coins. Finally, under the waterfall is a Warp Pipe which leads a room with a Painting leading to the rematch against Mollusque-Lanceur.