You visit the island only once, so look for treasure chests before you rescue all dwarves.
After a long search Geralt was at last a step from his goal. Avallac'h had hid Ciri from the Hunt on the magical Isle of Mists, which could only be reached if fate deemed one worthy - or by following a magic firefly, which was decidedly the easier option. Geralt thus readied himself to sail to the isle and bring back Ciri.