Start with the correction, because every visitor arrives needing it. Crab Island is not an island. There is no sand above the waterline, no shade, nowhere to put a towel and nothing to walk on. It is a submerged sandbar lying in Choctawhatchee Bay just north of the Marler Bridge, the span that carries US 98 over East Pass into Destin, and on a Saturday in July several hundred boats tie off above it while the people aboard stand around in water somewhere between knee and chest deep. That is the entire proposition. It is also, by a wide margin, the most searched thing on this coast.
The sandbar exists because somebody put it there. The Army Corps of Engineers began dredging East Pass in the 1930s to keep a navigable channel open for Destin's fishing fleet, and the spoil was pumped into the bay behind it, where it piled up into a genuine low island that birds nested on. Deposition stopped around the end of the 1960s, and a few years later Hurricane Eloise flattened what was left into the bar you wade on now. Accounts differ on the exact years. Nobody disputes the shape of the story: this is a piece of industrial leftover that became the region's signature attraction.
Here is the part most Crab Island articles skip, because most of them are written from Destin. You are staying in Fort Walton Beach, which puts you on the wrong side of the water. The sandbar sits roughly nine or ten miles east of the Brooks Bridge across open Choctawhatchee Bay. From a Destin Harbor dock it is a ten to fifteen minute run. From a Fort Walton Beach or Okaloosa Island ramp it is a proper bay crossing, and in an afternoon chop on a rented pontoon that can take the better part of an hour each way and be genuinely uncomfortable. So the first decision is not which boat. It is whether you drive east over the bridge and board in Destin, or stay on this side and accept the run.
The second decision is what kind of day you want. A pontoon rental gives you a base: shade, a cooler, a ladder, the freedom to sit there for six hours. A shuttle or a scheduled tour boat gives you a fixed window of two or three hours, a captain who parks the thing, and a restroom, which on a hot day with children matters more than people expect. A private charter adds the rest of the bay, and this coast carries an unusual weight of that inventory: more than a fifth of the bookable product here is priced above $250.
One honest warning before you plan around it. Crab Island in season is loud. Music from a hundred hulls, wake slapping between them, inflatables, vendor boats idling through the gaps. If your picture of a Gulf holiday is a quiet beach, this is not it, and the right answer may be to do it once for three hours and spend the rest of the week on Okaloosa Island. If your picture is a floating block party in waist-deep green water, it is the best thing on the Emerald Coast and the rest of this page is about doing it properly.
