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For swim qualifications do new midshipmen usually fail or is it relatively easy? I've never done the prone float before and I've never done the trouser inflation.
It completely depends on how comfortable you are in the water. The instructor will explain what you need to do, demonstrate it, then allow you to practice. If you are a strong/experienced/comfortable swimmer, you should not have issues. If you don't pass first time, not really a problem. Keep practicing!