The Lifeguard Hut Safe Key in Dead Island 2 can be found during/after The Giant-Slayer story quest. Travel to the southern tip of the beach as shown on the map below to find a named enemy.
Afterwards, travel to the nearby lifeguard hut and unlock the Venice Lifeguard’s Safe.
As a reward, you’ll receive the Tenacious Tactical Sword! For more guides on Dead Island 2, click here!