Tafsir of Al-Anbiya' 21:82

ومن الشياطين من يغوصون له

And of the Shayatin were some who dived for him, means, they dived into the water to retrieve pearls, jewels, etc., for him.

ويعملون عمﻼ دون ذلك

and did other work besides that; This is like the Ayah:

والشيطين كل بناء وغواص

وءاخرين مقرنين فى اﻻصفاد

And also the Shayatin, every kind of builder and diver. And also others bound in fetters. (38:37-38)

وكنا لهم حافظين

and it was We Who guarded them. means, Allah protected him lest any of these Shayatin did him any harm. All of them were subject to his control and domination, and none of them would have dared to approach him. He was in charge of them and if he wanted, he could set free or detain whomever among them he wished. Allah says:

وءاخرين مقرنين فى اﻻصفاد

And also others bound in fetters. (38:38)