Possession Sound is a substantial saltwater sound in Washington's Puget Sound, lying between the mainland cities of Everett and Mukilteo to the east and Whidbey Island to the west. It is approximately 3 to 6 nautical miles wide and 9 nautical miles long, characterized by shallow depths that amplify the effect of large tidal exchanges, a critical factor for trip planning. The sound is known for developing significant chop and whitecaps even in moderate winds, with 10 to 15-knot winds capable of generating 1 to 2-foot waves due to its long, open fetch. Afternoon winds are typical and can create challenging conditions. Currents within the sound are generally weak and variable, though strong currents can be encountered in specific channels, such as near the Everett Marina and the mouth of the Snohomish River.

Key public launch points include Lighthouse Park in Mukilteo, which has a boat launch and hourly paid parking that fills quickly in summer, and Port Gardner in Everett, a sheltered boat ramp suitable for beginners. Other access points include Edgewater Beach in Mukilteo, the 10th Street boat launch at the Port of Everett, and Howarth Park. A primary paddling route is an 8.3-nautical-mile round trip from Lighthouse Park across Possession Sound to Whidbey Island, a crossing of about 1.5 nautical miles, then south along the island's shoreline to Possession Beach Waterfront Park or Possession Point State Park. Paddlers must remain vigilant for frequent pleasure and fishing boat traffic during this crossing. Landing on Whidbey Island is restricted to public parks, as most beachfront property is private.

The sound is a renowned seasonal feeding ground from early spring through May for a resident population of gray whales known as "sounders," which forage for ghost shrimp in the muddy bottoms. Other common wildlife includes harbor seals, California and Steller sea lions, and diverse seabirds. The aesthetic is that of a broad inland sea with views of the Cascade Mountains and Mount Baker on clear days, featuring artificial structures like the line of sunken wooden ships serving as a breakwater at the Snohomish River delta. For safety, paddlers should be prepared for sudden wind changes, have immersion gear such as a wetsuit as a minimum, and plan trips around tidal stages, particularly near river mouths.