The closest body of water to Tibet would be the bay of Bengal, imo.
Also, the other portal locations except the Earth Kingdom aren't perfect. The Fire Nation is as Thai as it is Japanese imo. The Southern Water Tribe are obviously Inuit, so Canada and Northern Alaska make sense.
I'd throw the one to the Northern Water Tribe to Antarctica to be contrarian.
What's with the numbers also? The Water Tribe, the smallest after the Air Nomads, has 40 portals total, whilst the Earth Kingdom, which far outnumbers all others, has 13.