What's the difficulty? These are different seas. One is the Sea, and the other is the sea. You could make it so that the density of the water in the underground sea is simply higher, allowing the Garray to move through it as if it were land, while only Krakens can swim freely in the surface sea; the surface sea is too thin for others, and they are pulled back into the depths. Or, you could make it so that the Garray have lived in the darkness of the underground sea for so long that they need protection from sunlight when moving on the surface (including in the water) – a ship.