Kaʾanna brings the subject of a simile into relation with its comparison. Ḥudūja is the accusative noun of kaʾanna, al-Mālikiyyati its genitive complement, and khalāyā its predicate. غُدْوَةً specifies the time, morning.
The poet does not casually equate a caravan with a ship. The comparison is between camel-litters and large vessels. One crosses desert and the other sea; both carry passengers and advance with a characteristic rocking motion.