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While yes, you could use SVG today, for performance reasons (not obviously for water, but more complicated stuff like an actual city) raster files are both more efficient in terms of computation time and more consistent in rendering with the only disadvantage of file size tradeoffs. Not quite a concern for computers and dedicated navi systems (in fact Google Earth and the Google Maps apps for iOS and Android* uses vectors, probably not SVG though), but other embedded systems don't really have the luxury of including a proper 3D or even 2D-accelerated drawing (there's a basic chip for raster rendering and relatively fast path drawing but city maps usually contains many geometric paths that overwhelm the graphics processor).

* If you know Android Go (not Android Auto), the Maps there are raster due to hardware constraints.

Yes

https://caniuse.com/?search=svg

Yes, it's everywhere.