Every GTA map to date has been island, which is unrealistic, especially in GTA V.
The least they could do is have an invisible barrier of land on the outside of the map. They did it very well with Red Dead Redemption and L.A. Noire so they could easily add unclimbable mountains or something. Keep the ocean on the western side of the map, move Paleto Bay to the western side too. Because California doesn't have ocean on the north, east and south.
Look it's like this, in every GTA since III, the map has been made into an island to represent the portion of the area that is familiar to the protagonist. Even IRL, I ONLY know an 'x' amount of my state/city, so if I where to draw a map of the only areas within my city I know, the map would look a lot like an island with the 'water' areas being the unknown. Thus this is how GTA maps are meant to portray a region.
Even though the rest of SA actually exist and is most likely attached to V's island mass by land, we have only the represented area of the map that the protagonists would know (or even care about, that's why Yankton seems like there's an ocean between the states when really it's just water being used as a metaphor for unknown, unseen, or otherwise not cared about areas - unless otherwise noted as a lake or river, etc)
So the map of GTA V is just a piece of land "broken off" of the rest of the state for gameplay purposes, mainly because crashing your jet plane into an 'invisible wall' going full speed would just be the worst.... Also computer generated land isn't R* style, every inch better be hand-made or go to EA with that sh*t...