I have had 4 10-12 foot jon boats of varring widths (one I named the surfboard) and had never felt them to be unstable when moving around. In fact I could stand on all of my jon boats and rock them side to side to the point where water was coming over the sides and still not flip them.

Jon boats are very easy to modify but so are V hulled boats. If you have never done any modifacations before check out this web site:

http://www.myjonboat.com/

Lots of good ideas to look at (you may or may not want to buy the book) Good luck, a modified aluminum boat is a good way to get on the water.