Regarding the dive, that rock really is particular, it stands out so much I would not be surprised it's a local attraction. It might really be the place where young guys show their courage by doing a high jump. If so, a local expert swimmer would know about it too.
The fact the writer decided to go there might be because he read it in a guide or heard about it, after all he is good at remembering details, so he might even have heard the place from Nako herself.

Then there's the fact the guy did jump and did not crash into anything solid. We can assume she did follow the entire scene and had a good idea of a safe from obstacle area where she could dive. The main difference being that the head on diving gets you deeper, hence the inapropriate risk she has taken when jumping feet (and ass) first would be a lot safer. But it's far less aesthetic/majestic... so they decided for the beautiful version