Is this the 3pm flight to Plattsburgh on Direct Air? If so then you really need to be at the airport no later than 2pm.
Some things to consider...and that is all I'm saying:
Charlotte County Airport (PGD) is primarily a general aviation airport that has a few commercial flights a day. It is more about the Cessna 172s than it is about Boeing 737s. This actually can be a good thing, but it likely also means TSA only has one lane, which means you may need more time to get through to the gate. It is good that the airport is right off of I-75.
Speaking of I-75, most of your road time will be in Alligator Alley and there are tolls you'll have to pay.
No rental cars are available at the Port Of Miami. You have to take the rental agency shuttle bus from the pier to their car lot. Most of them have lots within a couple miles of the port, but still the clock is ticking.
Realistically, the best you could hope for is to be leaving the ship by 9am. It could be a little earlier but you are at the mercy of U.S. Customs & Border Protection. I've personally been affected by a computer outage at Miami, which shut down debarkation for 45 minutes since CBP is totally dependent on computers to process passengers. I strongly urge you to be in the first group to leave the ship.
Most likely you will be okay and will make it to PGD in time, but you don't need the little delays and glitches to start accumulating. Otherwise you'll find yourself spending another day or two in Fort Myers waiting for the next Direct Air flight.