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That's all five Zero Isles done, and my reward at the end of Zero Isle Center?
Four Wooper Bows, which protect from weather damage. Seeing as how Wooper's already part Ground, that just means hail doesn't hurt, not worth keeping in the bag. Was also told the treasure boxes that enemies drop were great, this isn't the case. Out of 2 pages, one box had an Iron and another had a Dawn Stone.
If you're going to go to Zero Isle, go North for Wonder Gummis or South for a shot at Tight Belts. Don't waste your time in the other ones.