點解自然旋渦轉的方向一定係逆時針方向

It is NOT true.

Rotation of the earth that gives rise to Corolis force only affects large circulation system, such as typhoons or monsoon winds, that spans over hundreds or thousands of miles.

For water circulation that you observe in your wash basin, bath tub or toilet bowel, the Corolis forces is too weak to have any effect on it.

Those kinds of water circulation are in fact governed by the flatness of the drain hole, the initial speed of water (as the water is hardly at absolute rest), the air movement above the water, plus other factors.

physics8801is wrong & kyfung8 is correct!! From 維基百科，地轉偏向力
存在條件 : 地球上非赤道地區對於地面擁有水準運動方向速度分量的物體
方向 : 垂直於物體速度的水準分量方向，北半球向右，南半球向左
簡易判定方法:北半球用右手 南半球用左手 手心在上 拇指與手掌撐90°角,其他四指指向物體運動方向,拇指所示方向就是地轉偏向力的方向.

因為香港位於北半球,而地球自轉既方向係由西轉向東,所以座廁沖水係逆時針方向。
如果係澳洲(即南半球),座廁沖水就會係順時針方向!

仲係唔明白既話,可以拎一個水樽,你不停咁將佢由西轉向東,你係上面觀察佢個轉動,就會係"逆時針方向",從水樽底部觀察,就會係"順時針方向"。