The Keys has the longest living coral reef in America, running 221 miles from Key Largo FL and continuing southwest to the Dry Tortugas National Park. The coral reefs protection keeps the waves from crashing ashore, which causes erosion. This also means the water inshore of the coral reefs is shallower, and calmer.

The remarkably calm waters surrounding Key West and the Florida Keys are primarily due to the presence of the Florida Reef, the only living coral barrier reef in the continental United States. This massive underwater structure, located about six miles offshore, acts as a natural breakwater that absorbs the energy of Atlantic waves before they reach the coastline. Additionally, the geography of the Keys creates a "backcountry" of shallow, protected lagoons and mangrove forests that further dampen tidal movement and wind-driven surface chop. Because the water is typically only 10 to 20 feet deep for miles around the islands, there isn't enough depth for large, rolling swells to form. This unique "shallow water" environment is what makes Key West a global destination for paddleboarding, kayaking, and light-tackle fishing. The calm conditions also prevent sediment from being stirred up from the seafloor, which contributes to the legendary turquoise clarity that defines the region’s tropical "look."