The Third Class swim test is typically administered early on in the fall semester, oftentimes in the first couple weeks. It may or may not be done during freshman orientation; that is unit specific.
The test is comprised of two modules. The modules are administered separately. Typically, the whole group will do module 1, and then the whole group will move on to module 2. Each module is pass/fail. Both modules must be passed in order to earn a Third Class qualification.
Module 1:
-Deep water abandon ship from a 10 meter platform into the pool below
-50-yard swim any stroke
-5 minute prone float (face in water)
The individual transitions from one event to the next in module 1 without stopping.
Module 2:
-Shirt/trouser inflation (basically, turn a pair of old camo pants into a life preserver using one of three demonstrated methods, and then float without motion for a matter of seconds)
That's it. The Third Class test is about safety, not proficiency. Therefore, strokes aren't evaluated for proper form. As long as the student can complete the test safely, they'll qualify. Students must qualify Third Class before they begin their sophomore year. Failure to qualify will typically lead to disenrollment.
Students will have the opportunity to take the Second Class test as well at some point, but it is not a commissioning requirement (it is required for service assignment to Aviation and NSW/EDO, though). The Second Class test is about evaluating swimming form and proper strokes. Proficiency vs. safety.