Key West is not a beach town. That needs saying before anything else on this page, because it is the most common mismatch between expectation and reality here.
The island sits behind a barrier reef. The reef is why the water is calm, why the snorkeling offshore is good, and why there is almost no natural sand on the shoreline. Most Key West “beaches” are small, imported, and modest.
Fort Zachary Taylor is the best of them, and it is genuinely worth a day.
What makes it the good one
Shade. Australian pines run along the back of the beach, which on a Key West afternoon is not a luxury.
Clear water. It sits at the southwest corner of the island where the channel runs deep close in, so the water is clearer than anywhere else you can walk to.
Actual facilities. Restrooms, showers, chair and umbrella rental, and a snack bar that is better than it needs to be.
The fort itself, a Civil War-era brick fort with the largest collection of Civil War armament anywhere, included with entry and largely ignored by people on the sand.
The rocks
The bottom is rock and coral rubble, not sand.
This is the thing every review mentions and every listing omits. Walking in barefoot is unpleasant and walking out is worse.
Bring water shoes. It is a ten-dollar problem that determines whether this is a good beach day or a short one, and shops all over Old Town sell them.
Timing
Cruise-ship days change everything. When ships are in, the lot fills and the beach gets a large temporary population that leaves again by late afternoon.
Weekday mornings are the quiet version. Late afternoon works too, and puts you at the southwest corner of the island as the light goes — which is a better sunset than most people fight for at Mallory Square.
If snorkeling is the point
Go offshore. The reef is several miles out and it is the real attraction - everything you can reach from the sand here is a consolation prize. That means a boat, which is covered on the tours pages.