I believe that breathing from a rebreather is easier than
breathing from an open circuit regulator. Therefore rebreather
divers have an advantage of reduced O2 consumption due
to reduced resistance when breathing. The bottles are better
positioned, providing greater bouyancy than with the single
open circuit cylinder or even the doubles, again reducing the
demand on O2. Physically, if an open water certified diver is
fit to dive, then that same rebreather certified diver is fit to
dive rebreather.