& one note coming from a Search & Rescue guy, that pulls abut 3 folks a year out of lakes from drowning incidents, please,please, please wear your pfds. Get a good one that fits well. There are also auto inflate ones that are lightweight, for those that want to stay cooler, or kayaking pfds that give you lots of room to move. There is no excuse to not wear one.
for sit in kayaks, most basic inexpensive plastic models are wide, some are short, and feel very stable. They are very stable in flat water, but do not handle rough water as well as a better model with multiple angles (chines) to the bottom hull design. A more performance oriented kayak maybe narrower, or feel a bit tippy to start with, but can handle rougher water much better with a bit of practice. If you only fish in calm waters, then the stable, wide models or sit on tops will work very well.