@Eggs. No one is copying anyone, and frankly this gesture was not even invented by the author of LTIT, but as I have written before here in the comments, it is a gesture that is used a lot narratively in stories because it invokes a precise symbolism concerning the human psyche that is often found especially in people's dreams: when we dream that we are diving into the sea or ocean, this is meant to signify that ourself wants to "dissolve." In fact, if you notice, Dan is not conscious and aware of such an action, he is being guided by his suffering and desires to "rest", that is to abandon his consciousness and dissolve himself in the water, which is why he unconsciously headed for the sea. This is because he is becoming familiar with the concept of death and abandonment, as well as that of guilt.
It is a completely different dynamic from LTIT, because there it is portrayed as a conscious as well as a desperate but impulsive gesture; it is nothing unconscious as in the case of Kim Dan, whose gesture is much deeper and driven by his unconscious.