Naples/Marco area is beautiful. Ferg/Foggy, I advise taking up Jim's (Jennykatz) kind offer to PM him re options, he lives there and knows all the ins and outs and is a very reasonable and experienced fellow C-Brat.
Be aware as you go down towards the Everglades it's very lonely; there is not much VHF radio reach and no cellphone service; it's not like Clearwater. I'd have a rented EPIRB from USBOAT or at least a PLB. Expect the usual northern Artic blast once a week or so to last for 3 days of Small Craft Adv conditions and be prepared.
I don't advise going to the Dry Tortugas in a C-Dory (even mine) based on a Fast Ferry trip there last Dec and a Seaplane trip earlier this month. Boris says there are 'millions' of lobster pot lines, but I say there are '10 million' and with typical wind waves you can't see the floats in the whitecaps.
Much more importantly, there is nothing much to see or do once you're there for 8 hours of snorkeling/fort tour. The huge cat pass ferry and 2 seaplanes are virtually next to you in the small harbor, no privacy at all.
You guys can take the Marco Island ferry to Key West ($164), then the Seaplane (highly recommended, $327 half day) or Freedom Ferry ($189 all day) to Dry Tortugas and check it out yourself.
Exploring the Keys by boat is much more interesting, even if you limit yourself to off the path beach bars that don't serve alcohol til 8am and do allow large wet dogs and sailors.
Key West is golden if you happen to be retired DoD. NAS marina transient slips are $1/ft, 2BR clean mobile homes on Trumbo Point for $70/night, 3 BR cottage with screened porch $120/night, Ft Zach annex beach right next to state park free, 10% discount at tony Stock Island Marina Village, etc etc. we'll be back every year!
Happy Boating and Happy New Year!
John