Translation:The patients were swimming for their health.
Thanks for the answer. True. I figured that old. I think Duolingo's German course literally translates German into English or maybe literally translates the expression of what German's would hear. For them this sentence would make sense and translation would be like that, maybe but understood normally like the way I wrote.
Does the preterite in German carry any significance distinct from the perfekt, other than formality? This sentence makes me wonder if there is a way to distinguish between ongoing past events (in English, the translation here "were swimming") versus point-in-time past events (which would have been translated "the patients swam...") Maybe there is no distinction in German, other than what is available from context.