It's a nautical reference. When someone dies at sea, you can't bury them, and you can't burn them, so you commend their souls to the deepness of the water. Spac

It's a nautical reference. When someone dies at sea, you can't bury them, and you can't burn them, so you commend their souls to the deepness of the water. Spacefaring borrows a lot of traditions from seafaring; the deepest ocean is the one above us.