Water also drains clockwise and foucault pendulums process clockwise in northern hemisphere. These are somewhat less likely to have a bearing on the matter than a sundial.
I've been to the equator in Ecuador. It was clever how the "museum" made everyone think that simply by walking 20 yards north or south equator that the water would drain differently. They had a rectangular basin with a drain in the center. When we were in the northern hemisphere, they poured the bucket so it would induce a clockwise swirl, and counterclockwise in the southern hemisphere. When were were on the equator, they just poured it right in the center so no swirl was induced.
This is poor science, and very weak support for 'definitely'.
I didn't watch this all the way through this time, but I do recall watching it back in 2017 when it was posted -- IIRC he did this experiment <sic>, asserting all variables were controlled for, but in a garage with leaky walls, on a poorly insulated floor, with no control test and only once, in one location.
Consider that hurricanes don't form within 5 degrees (~ 550km) of the equator - which gives a good indicator of just how strong (or not) the Coriolis effect is.
There has been experiments, published in Nature in the 60s that did confirm an effect. I personally know one of the authors.
The tub they used was something like 60cm deep and 3 or 4m in diameter.
I can pull up the reference if you like.
It wasn't perfectly reproducible though; in classic fashion when they demonstrated for a local newspaper (Sydney Morning Herald) the water went the "wrong" way.
I have given the url / details here in HN a few years back.
That it's not perfectly reproducible should be setting off alarm bells, even for people who would really like this effect to work at the scale of a few metres.
AIUI the math is that the delta in acceleration across a 1m bathtub north-to-south would be 1x10-8 m/s/s due to Coriolis ... so your starting conditions would have to be at a level of perfection that I don't think anyone's achieved, certainly not some bloke in a drafty garage with a kiddy's wading pool one time.
I accept that in theory that delta definitely exists, but I don't accept that it can be, has ever been, observed at the tiny scales of a few metres, let alone the 100mm (toilet flushes) that's typically asserted by the faithful.
I accept that across dozens of kilometres we definitely do see this phenomenon as it generates consistent, hemisphere-dependent, large wind events (eg hurricanes) once you get a good way from the equator.
It's probably reproducible in the sense that if you do it a 100 times you get a strong statistically significant effect. They were probably unlucky that day.
This must be one of the most misunderstood pieces of popular science.
The Coriolis effect is WAY too weak to affect the direction the water is flowing. Typically the way the water moves initially and the shape of the container will have an outsized impact in the final direction.
I think the issue is between people who interpret the statement as "water is predisposed to drain in a particular direction depending on hemisphere" or as "water will drain in a particular direction depending on hemisphere".
Does it have an effect and can it sometimes be seen? Yes. Is it often swamped by other factors and rendered irrelevant? Also yes. That doesn't mean it's false, but it does mean that some statements that rely on it as a factual basis are at best mistaken.
When you have a large reservoir with a drain, keep it still, and the water reaches the level where it starts to rotate, the Coriolis effect is the dominant influence.
It is something large enough to be plainly visible, but not something that appear on every drain.
Water also drains clockwise and foucault pendulums process clockwise in northern hemisphere. These are somewhat less likely to have a bearing on the matter than a sundial.