Sorry to revive an old thread, but there is not much out there on the Internet about this topic, and I want to clear up some outdated info:
The breath hold swim test does NOT require you to stay fully submerged underwater. I don't know if it did at one point and they changed it, but as long as you keep your face underwater, it is ok if you float to the top. Emphasis on BREATH HOLD.
My understanding is that they recognize that some people are more naturally buoyant, and want to avoid having divers weight themselves for a swim test.
Source: passed Fundies a few days ago. Had been unnecessarily worried about this, as I float right to the top...
Per instructor: "Technique that works best is a very slow breast stroke with long gliding phase to not overuse energy"
Helpful video: