In the Gravity Falls episode "Not What He Seems", we see a series of gravity anomalies in and around a lake.
Since it has the same density as the other objects, why didn't the water float away?
Maybe the lake was considered to be a compact mass( that means the whole lake is an object, not just a lot of water). Or this may be another error of the show like the ones showed here. We all know that the allusions of the cartoon make it more probable to human production error.
If it helps, +1.