I've been fishing out of a 13.5ft Sit-on-top (Ocean Kayak Trident 13 Angler) since 2010, including 3 months in Florida each winter. I got into kayak fishing after I retired at age 60, and I wish there had been SOTs 20 years ago. (I also have one of those expensive fishing boats.)
I've had a blast fishing from my yak, in small lakes, large lakes (Lake Huron, Lake Simcoe, etc), & both oceans on the east & west coasts of Florida & in the Florida Keys, catching a huge variety of species, including a 5ft shark. You get to fish in places that most guys never see, or in areas where even skinny-water boats can't go, and the fishing is always very up-close and personal, which is the biggest rush..
My advice is to buy the highest quality, most stable kayak you can, (nothing wrong with buying used quality) and one that you can load by yourself, so try it out first, for paddling and loading. Also buy the best seat and lightest, strongest paddle you can, and a rudder system - when you are faced with a long paddle against or across a stiff breeze you will know why those 3 things are important. Add an anchor trolly, which lets you anchor from the bow or stern. If you are new to kayaking, a re-boarding device might be also a good idea, although the only time I used mine was to test it in the first week - now its just a security blanket.
Don't listen to the naysayers - kayak fishing is comparatively cheap, great fun and good exercise.