The Conch shell is an item that can be earned through skilling or combat during the Port Sarim Invasion. It can also be found in supply caches. Only one conch shell, either from participating in the event or from supply caches, can be owned at a time.
When blown, it will provide a 'Conch!' buff that gives 25% increase in the experience from skilling activities in the event or one extra token per kill and 25% extra experience from killing the Skulls pirates. The buff lasts for three minutes. The shell is consumed upon blowing. This buff does stack with any bonus experience you have, and only one conch shell buff can be active at a time.
The 'Conch!' buff did not increase the experience given from using tools in the Boat to the Arc event.