I prefer a swimming pool because beach = the beach has a shark or jelly fish. Also, beach water is so salty. But, swimming pool = But a swimming pool has lots of kinds of water. Also, it is not salty. At this reasons, I like swimming pool more than beach. = For these reasons, I like swimming pools more than beaches.

I totally agree with your point! By the way instead of using "at", we use "for" as it indicates the purpose. Thank you!