Jin Yan dashed into the Star-water Lake because that would greatly increase his chances of escape.
Both of them would be tremendously restricted by the water pressure, while their vision and mental perceptivity would also be severely restricted by the dark bottom of the lake.
With Su Mo's cultivation, he was considered good if the range of his perceptivity could even reach half a kilometer in the depth of the lake.
But it was different for Jin Yan. As an expert in the Enlightenment Realm, he had already developed a spiritual consciousness that could detect a range of two and a half kilometers normally. Even at the bottom of the lake, his spiritual consciousness could easily extend over one kilometer.
As such, he could keep track of Su Mo easily, while Su Mo might not be able to do so. This gave him the confidence of escaping.