Today’s Sims 4 game patch added the ability for Sims to wade in random bodies of water across different worlds. Among the numerous wading spots in the game are Willow Creek and Magnolia Promenade, where Sims can get knee, or even waist deep.
Wading spots are typically located in areas where fishing is available and the terrain is low enough that Sims can walk right into the water. Just look for the “Wade Here” interaction when clicking on the water, and Sims will head right over. Full blown ocean/lake swimming is only possible with the Island Living expansion for now, but perhaps Gurus aren’t too far off from making it possible in worlds across The Sims 4.