This is all just my opinion of course, but I think your best choice boat for off shore fishing and sailing is indeed the Adventure Island type boats. An Adventure Island if your alone 90% of the time is a very good choice, and a Tandem Island if you want to bring along friends and family, The TI is a fantastic family boat with the trampoline option or HAKA's you can take your whole family out. We have often had 3-4 people on our TI (adults and children). It has two regular seats (front and back), and with the tramp option you can carry a couple more if needed (we do it all the time).
If you were an inland fisherman of course the PA is the ultimate fishing platform, but I understand it's not as stable offshore as most people would be comfortable with, basically you would be in trouble in 2 ft surf and waves, though some do it, most would prefer not to.
Regular kayak fishing off of an Oasis, Revo, or Outback is also very popular (we started out kayaking on the Revo and Oasis and enjoyed for many years, but mostly very close to shore, and/or inland protected water areas. I'm sure there are guys who take their regular kayaks offshore into the ocean, but you have to consider that the ocean is huge, and to travel any distance to get where you need to go so you can begin fishing (sometimes 5-10 miles from launch).
I think the preference is to be able to get out to where you want to be as quickly as possible so you can spend as much time as possible actually fishing or whatever you want to do (we are divers and we spear fish, we don't rod and real fish much).
With the tri power ability of the Adventure boats ( wind, pedal, paddle, or alternately motor) you can get to where you want to be quickly in pretty much any conditions and travel safely pretty good distances, whatever the conditions. We often take our TI offshore and travel sometimes 15-25 miles away from launch (not straight out into the nothingness of open ocean of course, (what would be the point)), but we can island hop down in the keys and the inter-coastal pretty much anywhere we used to take our powerboats. And best yet you don't need any previous sailing experience at all to have fun, they are the easiest boats in the world to sail, plus you have the pedal drives (if the wind dies).
Personally I have never been to the area around Ft Pierce, we stick pretty much to the southern gulf coast and the keys, as we have houses in both Sarasota, and Key West, where we try to spend time at both when we can.
My Brother just moved to Port St Lucie, and we have already talked about me taking my TI over there to explore (and he does fish). He heard that the St Lucie river and the St Johns rivers were both very nice, though I'm pretty much salt water person (because I'm Canadian and I terrified of alligators mostly), we will probably be exploring around there sometime soon. Actually hopefully someone from that area will key in here and give us both good ideas on where to go in that area.
Good luck on your trip and hopefully your upcoming purchase.