Damon swam underwater with the goal to put some distance from himself and the strong currents of the waterfall.
It's been a while since he did a stunt like this one, and he found it cathartic.
Was there a chance for him to die by jumping into this waterfall? Probably not.
At most, he would get badly hurt.
Why did he take that risk? Because the freefall mixed with the uncertainty of what's hiding below the mist, and it was exhilarating.
For a control freak like Damon, jumping into the unknown was stressful and addicting at the same time, and it rewarded him with an immense sense of accomplishment when he emerged out of it unscathed, making him feel like he gambled with death and came out victorious.
Just like every time before, the rush of adrenaline purged all insignificant things from his mind. In this case, as he was falling down, there was only one thing on his mind. Talia.
