If all you want is a kayak camera get a II.
Stay away from the III as most used ones will have a cracked plate or you will crack it.
I have a V... VI's and V's have many of the same issues if flooded...
Buy what you can find at the price you want... KEH has a nice selection.
This guy on e-bay is a straight shooter and doesn't sell junk.
http://myworld.ebay.com/ebaymotors/ibeendivin
Artsy underwater... I would expect you will want a 20mm
PS I actually use these things.