I'm shocked that all these answers are well above 50%. I wonder if you would have been thinking it was so high a percentage, if I hadn't called attention to it.
Inland courses from the GOLF Magazine top 100 list:
Four of the top ten: Pine Valley, Augusta, Oakmont, Merion
Nine more in the next twenty: Sand Hills, Royal Melbourne (close to 3 miles), Pinehurst, Winged Foot West, Chicago Golf, Prairie Dunes, SFGC (also close), Kingston Heath, Oakland Hills
Eight more of the next twenty, so that's 29 coastal [58%], and 21 inland of the top 50.
Out of the top 100, it is still right around the same, 57 or 58%. I wasn't sure whether Royal Adelaide or Valderrama was close enough to count. Olympic (Lake) was really the only course in the top 100 that's inside three miles but not very firm & fast.
I would agree with Peter (to some extent) that soils and topography have a lot to do with this. But I also believe that views of the ocean count for more than we all want to admit. In fact, I'm pretty sure of it, because I know how my own courses are rated, and I am not convinced that being able to see the water is a critical factor in being able to do my best work.
P.S. There is a large % of the population of the world that lives near large bodies of water, but it cannot be anywhere near 58%. And what % of all the golf courses in the world are close to the water? Again, nowhere near half.