Our swimming lessons are basically teaching commands to the babies. To get them going under the water you have a command "name, ready, go" then wait a few seconds before putting a handful of water over their face, stroking your hand down their face at the same time. Practice in the bath, 3 times each bath time, and do it every time you swim. They learn it, and to expect the water, so hold their breath. We did it for about 3 weeks before the first time we put the babies under.
It's also important to commit to putting them under the water, if you do it too slow, or too shallow, their gag reflex doesn't kick in and they'll swallow the water.