From Bulbapedia, the community-driven Pokémon encyclopedia.
Turtleshell Pond (Japanese: こうらのいけ Shell Pond) is a Friend Area in Pokémon Mystery Dungeon: Red Rescue Team and Blue Rescue Team that can be bought from Wigglytuff in Pokémon Square, or it can be a starter area if the player begins the game with Squirtle or Psyduck, either as the main character or the partner, though in Psyduck's case, it can only be the main character. It costs 5,500 when purchased from Wigglytuff. It is in the Pond area, and has a capacity of ten Pokémon, all of them Water-type.
Turtleshell Pond is a small, clear, bubbly pond. It appears to have turtle shells floating in it, but they may very well be oddly-patterned rocks. It also has a very large rock, or possibly a small building, rising out of the water in its approximate center. As opposed to walking on the water, the Pokémon there stay on the boardwalk between the grassy shore and the rock/building in the middle.
A small pond with a turtle-shell pattern. Pokémon that gather here love to play in the water and rest on the island.