They have manual versions as well. They won't inflate until you pull the rip cord.
Mine is an automatic, so if I hit the water, it inflates. And yes, I have already been embarrassed by my PFD once. My friends got a good laugh when I lost my balance in 3 feet of water.

Personally, I like the automatic. If I pass out when I am alone (or slip and hit my head) I don't have to worry about anything.
This is what always bothers me about people who say they don't need a life jacket because they are excellent swimmers. Your swimming skills won't serve you very well if you aren't conscious.