If someone wants to dive, and cannot do your "float test," by either treading water, floating, or using the drownproofing technique, not only should that person always wear a life preserver around water, but also that person should not be diving! That is an accident waiting to happen.
The funny part is that pool day 1 was the "try it" dive and both of them swam happily underwater in gear. Perhaps they shouldn't have been diving, but evidently they could. Dive: yes, swim: no.
I, OTOH, feel safer swimming underwater holding my breath and a diving brick than in a car is some places/with some people driving.