Dude...its SUPER easy. I agree w/above. They'll help you if you show signs of struggle. But seriously, i was a sh!tty swimmer when i showed up to nrotc (I couldnt even tread for a 10 seconds). 3rd class isnt that hard. Its all mental. Just stay calm and if u feel like youll be shallowing water, just yell TTO (or whatever you scream when you feel like dying) and the instructors will probably yell at you, then help you. But again, im speaking from Middy exp. vice OCS exp.