The boy is swimming at the pool.为什么是at the pool?用in the pool

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“在池边”的意思吗？"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！因为在游泳啊。这个句子可是初中课本上的原话啊。"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.