I think the word undertow can be a bit misleading. I think the name derives from the fact that this type of rip is below the surface. It doesn't actually mean that you get pulled under water by it. Undertow is just another word for a rip. People who can't swim standing on a sandbank and then get pulled off the bank by the rip will go under but not people who can swim.
I've been in the sea almost all my life and never come across a rip that pulls you under. The only thing that will ever pull you under is a suck up from a wave. And occasionally in big surf I find that sometimes in the whitewater after a big clean up set that you can get pulled down a little by the wash.
EDIT found a link here: http://www.ripcurrents.noaa.gov/glossary.shtml