Swimming lessons will be your best bet.

You might also try getting her some fins to try out (you can get cheap fins
at Academy or Big 5 Sports or similar places), but I'd recommend the
swimming lessons first.

The problem is the way your feet point.  Most sports, and walking in
general, tend to place your ankles at 90 degrees.  Now, look at your feet,
sit in a chair and pretend you are kicking.  When your foot moves up, the
angle of your foot (the top of your foot) is moving you backwards in the
water.  When your foot moves down, the angle of your foot (the bottom of
your foot) propels you forward.  So, you stay in place.

Good swimmers have very flexible ankles, and are able to point their feet so
that either direction propels them forward.  Fins are flexible, so no matter
how you kick they propel you forward, and some fins are even on an angle to
compensate.

To pass a scuba swim test you only have to navigate across the pool, with
any stroke and with no time limit.  Since scuba and freestyle swimming have
little in common, keep up with the swim lessons but use another stroke for
the test.  The main thing is that she is comfortable in the water.  The PADI
swim test lets you use a snorkel, mask and fins.  I'm a lousy swimmer, and
passed both the PADI and NAUI swim tests.  Like your wife, without fins and
doing a freestyle stroke, I kinda don't go anywhere either.  My best stroke
is a side stroke.

When I was a kid I was on the swim team.  Apparently my ankles stiffened up
from other sports.

Put some fins on her and see how she does.  There are a lot of people that
don't get much power from their kick and still swim, and dive, just fine.

Lee