Although many people refer to the float requirement as a "tread" requirement, it is not so. Your job is to stay afloat for 10 minutes. That can be done in many ways, including (but not restricted to) treading water. Backfloats, drownproofing--all are good. In fact, the primary requirement is that you don't drown for ten minutes. If you drown during this test, you automatically fail the course.
Ah, but are you allowed to retake it?