Once you have the water squirter just go around spraying water on anything that has butterflies flying around it; usually flowers or small trees. Each item is categorized into a group and once you complete the entire group it will count as one secret. Just because plants look the same doesn't mean they are in the same group like the island flowers and the tree house flowers in Adventure Land, so squirt all of them. Also, some groups have multiple kinds of plants in it like the New Orleans flowers. Once you do this in four or five lands the achievement will unlock, but I would recommend avoiding Fantasy Land and Tomorrow Land because each group has way too many plants in it.