Paul,
 
Swimming budgies?  Actually they can swim after a fashion -- mostly
float with a lot of useless splashing.  They can't keep it up for long
as they tend to tire themselves out, become water logged, and then good
bye little bird.  As for launching out of the water, it seems to be an
incident specific deal.  I have seen birds hit the water, splash, and
then take off.  I have also seen them hit the water and just flounder
around.  Both cases were red-winged black birds hitting the water to
avoid a Cooper's hawk attack.  It could be the one that flew off was
really doing more of a bounce and go rather than actually getting in the
water.
 
I don't believe any of these small guys would ever enter deep water
intentionally.  We have a bird bath and a swimming pool in the back
yard.  Occasionally when we forget to fill the bird bath (a daily
requirement in this desert climate) the birds will congregate around the
pool.  They will stare intently at the water and even fly down close to
the surface but I have yet to see one actually land.
 
We also have several birds (parrots and such) in the house.  All of
them  love to bath but none will venture into water over a few inches
deep.
 
For what its worth,
 
Tom Eichhorst in New Mexico, USA