The Sea God's Tear is a Rare item in Golden Sun: The Lost Age. It is found in Daila's Sanctum, held by the big statue, but you cannot get it until later in the game when Piers has joined your party so you can use Frost to be able to reach it. The Tear is to be placed in the hands of a similar statue within the Shrine of the Sea God dungeon, which causes it to elevate you to the upper floor where the shrine's main prize, the Right Prong to the Trident of Ankohl, awaits. After you obtain the trident piece, you can sell it for 225 coins and remove it from your inventory. If you for some reason need it again to get to the top of the dungeon, it may be repurchased for 300 coins.
Also inside the Shrine of the Sea God is the Rusty Staff, which can be fixed by Sunshine at Yallam to be reforged into the Glower Staff which has +126 Attack and a good Unleash.