From the beach warp, go south five screens(by using the walkable offscreen tiles), which should bring you to a corrupted screen. Go west one screen. Move to the sixth tile from the right and go north one screen into the sea and you will appear right in front of the chest. Alternatively, before entering the sea screen with the chest, position yourself on the left most tile of the corrupted screen and then go north to the sea screen with the chest. You can then take the off-screen tile on your left and swap it with the sea tile on your right to create your own path to the chest.
A video of an alternative way to get to this secret is here.