I already knew he was making a dingy and also this car with huge fat wheels that can cross a creek. https://www.youtube.com/watch?v=XIbZCtqBXMg
It looks like the coast of dry rock island to me. with stretched cargo container ramp added by him for use in the video. Edit: Oh, just noticed later in the vid you mean the creek/river place.. that is East Coast.. I know those places, one can be found by driving backwards up the road you spawn at the other forwards below the first bridge.