There’s an article that have a brief introduction about the sightseeing bus, unfortunately it’s in Chinese but you can try google translate
???????????????????????However, from what a seem in the article, the trip looks not that attracting, and probably not convenient. Considering the fact that Xiamen downtown area is small and it only takes less than $15 for a taxi ride from airport port to most places, I will recommend a self organized trip if you are plan to spend a half day in Xiamen