One of the only ways to get to the Sky Islands, the Sea Cannon is a stone square underneath Cumulus Island, Southwest of Savaria, which fires anyone standing on it high into the air every 2 and half minutes (contradicting the NPC there, who says "about 5 minutes"), causing you to get launched into the air next to Cumulus Island where you have to get on before you fall.
As you get shot into the air, you become immobile which can be fixed by jumping in the air. If you were waiting for the cannon to launch you up and got caught off guard, you can use a slow activating ult while at the highest point to buy yourself more time to land on the island. Similarly, slashing your sword in mid-air can also buy you some time to slowly land on the island.
Near the Sea Cannon, there are multiple, smaller "Sea Cannons" lying dormant, underwater.