Chumley's Shell is treasure that can be obtained in Sly Cooper: Age of Thieves in the episode Coopers of the Caribbean. It is found in the episode's vault and upgrades the Cooper Gang's stats, specifically Bentley's.
"While Mz. Ruby's old lackey, Chumley, may be long dead because of his master's actions, his shell provides those wearing said shell with the valuable ability to not drown in deep bodies of water, be it fresh water or salt water. Bentley also has his defense greatly improved by this shell."
The vault that this treasure is located on the Emerald Eclipse directly in front of the helm. The code to access the vault is "6-5-7-4-4", and the passwords needed to open the vault are "Gemstone", "Ocean", and "Buccaneer".