Gannon Island: Keewaydin's Backwater Marker

What Is Gannon Island?

Gannon Island isn't a place you'll ever step foot on — and that's exactly the point. It's a small red-mangrove key sitting inside the winding backwater near the south end of Keewaydin, one of a string of mangrove islands and cuts that boaters and captains use to read this water like a trail sign. Ask ten visitors what Gannon Island is and most won't have heard of it, because it never shows up on a highlight reel. It shows up on a chart.

Like nearly every small key scattered through this stretch of Rookery Bay backwater, Gannon Island most likely built itself the slow way — red mangrove prop roots snagging sediment, shell hash and drifting organic matter in the shallows until a scrap of land held together and grew its own thicket. No sand beach ever formed on top of it, which is why it never became a stop the way Keewaydin or the nearby barrier islands did. It's mangrove root to mangrove root, tide to tide.

Where It Sits on the Chart

You'll find Gannon Island inside the waterway near the south end of Keewaydin, in the winding inside passage that connects toward Little Marco Island and the mainland. It sits close to marker 28A, one of the green day markers boaters lean on to stay in the cut through this shallow, braided water instead of running aground on an oyster bar or a falling-tide flat.

If you've spent any time on the water down here, you already know this stretch doesn't run in straight lines. The channel doubles back on itself, splits around mangrove keys, and narrows in spots that look wider on a chart than they feel from the wheel. Gannon Island is one of the visual cues our captains use to confirm they're still in the cut — not a destination, but a checkpoint on the way to one. It sits in the same general neighborhood as Capri Pass, which carries boats out toward open Gulf water on the other side of this backwater maze.

Why It's a Marker, Not a Stop

Every boat-only stretch of the Ten Thousand Islands has places like this — islands that matter to navigation more than they matter to a day trip. Gannon Island has no beach to walk, no shell-strewn point to comb, and no dock or landing of any kind. It's mangrove all the way to the waterline, which is exactly what keeps this backwater wild and exactly why you won't find a tour that stops here.

That doesn't make it unimportant. In water this shallow and this twisted, knowing your markers is the difference between a smooth run and a grounded prop. Our captains use Gannon Island the same way they use dozens of other small keys through here — as a landmark that confirms position, not a place to anchor up. If you're running your own boat through this stretch, it's worth knowing Gannon Island by name and sight for the same reason: it tells you where you are in a maze that otherwise looks like an endless run of green tunnels.

What You'll See From the Boat

Even without a landing, Gannon Island earns a look. The mangrove fringe here is classic Rookery Bay habitat — prop roots working as a nursery for snook, snapper and other juvenile fish, and low branches that wading birds like herons and egrets use as everyday perches. You'll often catch an osprey working the edge or a pelican loafing on an overhanging limb as you idle past.

In winter, when water temperatures drop below about 68°F, manatees push into protected backwater pockets near Rookery Bay to stay warm — not in the big herds you'd see up around Crystal River, but scattered individually or in small groups tucked into calm, mangrove-lined water like this. Bottlenose dolphins live in these waters year-round and turn up on the inside route often enough that our captains keep an eye out passing Gannon Island, though we never promise a sighting — that's the Gulf's call, not ours. What you can count on is the scenery: a genuinely wild slice of backwater, the kind of place that makes it obvious why this stretch of coast stayed undeveloped.

Which Tours Run This Water

You won't book a trip built around Gannon Island itself, but you'll likely glide past it if your captain runs the inside route toward Keewaydin. Our shelling tours use this backwater to reach Keewaydin's boat-only beaches, where empty-shell collecting is the rule in Collier County — live shells, sand dollars and starfish stay in the water. Private charters are the best way to slow down here, since a custom trip lets you idle through the mangrove cuts, point out ospreys and wading birds, and let your captain explain exactly what you're looking at and why the markers matter.

If your interest runs more toward wildlife and open water than shelling, ask your captain how a given day's route lines up with this stretch — the inside passage through here connects toward Little Marco Island one direction and out through Capri Pass the other, and which way you go depends on tide, wind and what the day calls for. Either way, spotting Gannon Island from the deck is a small reminder of how much unmarked, undeveloped backwater still runs through this part of Florida once you get off the main channel. Head back to the full Destinations guide to see where else the inside route can take you.