The Acuario Mazatlan opened in 1980 and was for years the largest aquarium in Mexico. It is a block back from the Malecon at Playa Norte and is the obvious place to take small children. The tanks cover Pacific reef fish, freshwater species from the Rio Presidio, and a shark tank with bull and nurse sharks; outdoor enclosures hold crocodiles, sea lions, and turtles.
A larger replacement aquarium, the Gran Acuario Mar de Cortés, opened in 2023 a few kilometres south near the cruise terminal and is genuinely impressive — bigger tanks, walk-through tunnels, and a focus on the marine life of the Sea of Cortez. If you can only do one, do the new one.