CathInfo.com

 It makes sense that water always seeks its own level. Water is a very heavy substance.

.
The heaviness of water has nothing to do with the first statement. Only under particular conditions does water seek its own level. There must be a confinement, for example. Water unconfined and in a free-fall environment does not seek its own level, but floats around in the air eventually forming a sphere or a globe. This globular action is due to the cohesion of the water molecules as well as the inherent surface tension , and it happens to any liquid substance in the same situation, none of which "seek their own level."
.
Surface tension is a phenomenon which causes the meniscus in small samples like a drinking glass or a test tube, under different conditions such as those found on the surface of the earth, such as a common laboratory.
.
These substances, such as oil or mineral spirits (which are lighter than water) also form a globular sphere, and do not seek their own level; so do mercury or liquid iron (both much heavier than water). 
.
Therefore the statement that "water always seeks its own level" is false.
.
If this were your answer on a basic physics class test you would get marked off for being incorrect.
.
Flat-earthers would do really poorly in any basic physics class. 
.
There are no physicists who believe in a "flat earth." They would have to be literally insane first.
.

I never realized that there was such a strong belief among some traditional Catholics that the Earth is flat. And yet, numerous commercial direct flights going from Australia to South America take place all the time, running within a bit more than half a day's time and no mid-air refuelling.

.
Flat-earthers have posted here claiming that no direct flights between S. America and Australia take place, and have shown there are videos where this proposition is promoted. They have said that a flight for example from Argentina to Australia crosses over some city in the mid-east (Arabia as I recall) as a refueling stop. They claim this proves that the route they're taking is a much straighter line than it would be to take a route that crosses the south Atlantic and the Indian Ocean.
.
(Obviously, this S. Am. to Austr. is a flight of longer distance and going the opposite direction from the one you mention:  Austr. to S. Am.)
.
However, they have failed to recognize that the shorter Great Circle route for this journey would take them over the South Pole region which is not temperature-friendly for commercial jumbo jets, and the fuel consumption (partially due to the need to heat the aircraft while flying through such a cold zone) would necessitate a re-fueling stop in Antarctica, which would be extremely expensive, if not well-nigh impossible.
.
The problem reduces to the flat-earthers' refusal to recognize the reality of any Great Circle route, since they claim that the earth is flat and THEREFORE no such "Great Circle" can exist.
.