On its most basic level, "The Swimming Lesson" by Mary Oliver is about a time that, as a child, some well-intentioned adult had tried to teach her how to swim by simply tossing her in and letting her struggle by herself to resurface and reach the shore. She describes this experience in dramatic, intense words to portray the shock and horror of that episode: "icy kick," "coughed," "waves reaching around my life," "lonely fall," "frenzied rising." The middle stanza refers to a "medieval maxim," which is referring to how back in the medieval days, people used to toss suspected witches into the water to see if they would sink or swim; if they sank, they were innocent, if they swam, they were a witch and were taken to be killed. It was a rather ridiculous and ignorant practice, just as an adult tossing a small child into a lake and expecting them to magically learn how to swim is also an unsafe and ignorant practice.
Oliver concludes by stating that she learned less about swimming that day, and more about how to survive anywhere--if she can survive being spontaneously tossed into water when she doesn't know how to swim, then she can also survive most anything else that life decides to throw at her.
If you analyze the poem on a more symbolic level, the entire "swimming lesson" analogy can be compared to life. We are tossed into life and told to live, even though we have no instruction manual, no previous experience, and no warning. And, living sometimes feels just like that first plunge into water--it too feels cold, icy, frenzied and confusing. But, as we learn to survive everything that life tosses at us, we learn to "survive in any place."
I hope that those thoughts helped; good luck!