How does someone create a snapmatic picture that looks like this?

[shmok2]

source

 

[REXX]

Waterfall.

[Sanches]

Are you asking for the whole process? Just simply record your gameplay with in-game record function (F1 button on PC) and then you can edit it in Rockstar Editor.
If you're asking about that water effect, then it could be a mod, he just tilted this model a bit and put him into the water. Or this is just a waterfall, just like Rexx told.

Edited October 17, 2017 by Sanches