Take a look at:

http://www.boatsafe.com/nauticalknowhow/pfdbasics.htm
http://www.pfdma.org/choosing/types.aspx

If there is a chance I might get knocked out, I would go with a Type I. If the goal is to encourage people to wear them, and you aren't doing anything risky (relatively calm waters, no rocks near the surface, not too cold), then self-inflating (or even manually triggered) PFDs will probably work well.

Disclaimer: I'm not an avid outdoorsman, so your mileage may vary.

Da Bing