I went swimming at/in a really nice swimming pool.

Yes, "in" pool refers to the pool as a place, including the surrounding environment and facilities. So you can say, for example:

On the other hand, "pool" in the pool refers to the pool itself (including water). So:

In your writing, I think that the author also uses the "surrounding environment" to use "in a beautiful pool": they may be swimming in the actual pool, but he is also here good friendly It means that there is an atmosphere. Facility

When I was twelve years old, I took a vacation with my cousin, one of them pushed me into the pool. Ever since, I am afraid of water. There is not much swimming pool in Rakka of Syria where I am from. On the weekend you spend your vacation with family at Ja'aber Castle near Rakka, or you can have a vacation at Lattakia on the coast, but do not swim. I am always at the beach. I decided to leave Syria, but I have not had plans on where to go or how to get there. My father was not enthusiastic, but in the end many of the people we knew had gone away and we somehow got used to it. I was thinking about going to England because I can speak English and understand the country, but I can see it far away. Most of my information comes from other Syrians who plan to travel to Europe.

Ever since, I made a big progress. I can not swim barely from the end to the end of the Olympic size pool, and I can swim many times. Now, I am doing more exercise than any other movement. Several weeks ago, when I was swimming with my wife, she also swam quickly. But one thing that I can not do yet is to fill my face with water. I noticed that it sounded foolish, but I would rather be noisy at the pool than swallowing water. I know this will slow my speed, but swimming is not a drowning but a little step for me. I really feel very good about my progress. Until I talked with this random person, he insisted on sharing some aggressive advice I did not ask.