in 也可以.
If the question is "Where is the boy?",the answer "at the pool" might be better,because "at the pool" implies a location.at the table 是“在桌旁”的意思，那at the pool 不是“在池邊”的意思嗎？"At the pool" means the small area where the pool is. "in the pool" means the boy is swimming now inside the pool.那in the pool 不是更好嘛！因為在游泳啊。這個句子可是初中課本上的原話啊。"at the pool" has also the meaning "the boy is swimming" now; however, it could mean the boy is on the side of a swimming pool. In the United States, at and in can be interchanged in many situations; there are only subtle differences between them.