Translation:A girl is swimming in a pond.
I think both lake and pond should be an acceptable translations for 池 (いけ). There's a big overlap between their usage in English. While lake can refer to something bigger (like Lake Michigan), most of the time the terms are synominous in English. If you look at usage rates in American English, the term lake is used 3x more often than the word pond (based on Google Ngrams). In Japanese it's the opposite with 池 (いけ) being by far the more common term to refer to a body of water over 湖 (みずうみ). There may be less overlap in the meaning of the Japanese terms, but if you stick two people in a city park and ask them what that body of water is called, the American will more often than not say it's a lake and the Japanese will more often than not say it's an 池 (いけ).
"Wa" marks something known that is being commented on. "Ga" marks something as part of the new information in the comment. "Wa" would prabably be translated "the (girl)" and "ga" by "a (girl).
"Wa" would make the sentence be about the girl and "ga" indicates that there is some broader focus to the sentence. Both sentences are grammatical
女の子「は」いけでおよいでいます - "As for the girl, she is swimming in the pond". Answers the question, "What is the girl doing?".
女の子「が」いけでおよいでいます - "A girl is swimming in the pond". Answers the question, "Who is swimming in the pond?".
が is emphasizing the girl as the subject of the sentence, while は is merely a 'general topic'. In many cases, they are interchangeable.