"I don't think a rope would last long out here, so a chain's all there is to find." Or a cable, perhaps? Wire? Still, a chain is the closest metallic thing to a rope and the easiest thing to use for their purposes.

He's not squeamish about getting sand on that pretty white coat of his, is he? Especially since there's an ocean right there for washing it off. There's not much room around where's digging at the sand, so Indigo stands back until he starts pulling. Then she joins in, planting her feet and pulling on the chain with all her might.