There are many oils heavier than water. In a salt water environment, where water is 7 API, it would remain above most common residual oils (Number 6 or Number 5 oil in common parlance.) There are heavy refinery cuts reaching down to negative 4 API.
La Brea (and other tar pits) may have lured animals in by being coated with a layer of rainwater, and what looked like a pond was not...