Timing wise, any time between late April and mid-June works.
Assuming you can get a charter
boat company in St. Pete to agree to allow you to take a boat all the way to the Dry Tortugas, taking the ICW
all the way to Marco Island would be about as much fun as a root canal. Frankly, you'd do better to charter
a boat in the Keys for that or, charter a boat in Ft. Myers for crusing there abouts.
N'any case, absent experience on the West Coast of Florida and particularly negotiating the passes, your best course from St. Pete would be to the north side of the Manatee River or Jewfish Key the first day; departing there early the next morning going outside to Venice the next day, spending the night at the Crows Nest; departing there the next AM and going outside to Cabbage Key via Boca Grand, timing your departure from Venice so you can make it through the pass at slack water
(or if that timing doesn't work, going inside on the ICW
through Lemon Bay and Gasparilla). You can spend the night either at Cabbage Key or on the hook at Useppa directly across from Cabbage Key. In either case, leaving early from there you can make the run through Pine Island Sound, across the miserable mile, and blow through the Sanibel Island Bridge and down to Naples or Marco if you can't handle night sailing (or directly to Key West if you can). From either of these you can go direct or head down to the Little Snake River for a night on the hook and then make an easy passage
to Key West (we like the Galleon Resort for it's amenities but there are other somewhat less costly places to stop). From there its a long day trip to Garden Key but doable so long as you leave KW at sun up an don't dilly dally. Otherwise you can stop at Marquesa Key and spend the night on the hook on the West side in 13-15 feet of water
. From there it's an easy sail to Garden Key.
Heading back we usually shoot directly from Ft. Jefferson to Sanibel Island and spend the night tucked up under York Island if the tides permit
an easy crossing of the Miserable Mile or, if not, at a spot just north of Punta Rassa out of the main channel at N26 30.096 W82 00.711 or abouts. From there back to Venice (long day); and from there back to the Manatee River (another long day).
The foreoging itinerary works but you're looking at quite a few stops and a fair amount of especially if your weather window's are not favorable every day. When we simply want to get to KW without fooling around we just go direct. From the Tampa Bay sea bouy to the Galleon is about 200 miles and that usually takes us about 30 hours. We try to pass the TB sea bouy at first light and figure on getting into KW around early afternoon the next day. The advantage of this is that we can usually figure on a pretty goood wearther window for 36 hours and we're usually far enough off to avoid the squalls that plague the coast in the early evenings. Not always, but usually.
There's a lot more to the story but this should keep you thinking, eh?