In todays technology (actually since 50's) having radars and SAM sites on water haven't been that big of a deal... Just like oil rigs such facilities can float on water... Of course it certainly is much much more expensive to do this... But for the sake of range and efficiency it is done.
The game can mimic this...
And on an unrelated matter I would like to see sam sites get upgraded to being "defense turrets". That way I can mount missiles or other weapons as I see fit...