Without delving into the intricacies of micro-organisms, something of which I am thoroughly ignorant, , Kefir is a milk-based ferment. "Water"-Kefir is not. The microbial compostions are extremely different.
The similarities in that they both look like some kind of "grains":
View attachment 481122
Else they are vastly different microbial culture...
The original names for Water Kefir are Tibicos, Beer seeds
See this thorough Science Digest articles about how different they are: "
A comparison of milk kefir and water kefir: Physical, chemical, microbiological and functional properties"
Interestingly enough , you could ferment milk with "water Kefir" aka Tibicos grains. It will produce a sour beverage, similar but quite different from true Milk Kefir but such culture is not strong and will eventually die-off... with each subsequent fermentation cycles..
OTOH both cultures in their "natural" medium (Water based medium for Tibicos ,like Sugary water, Fruit Juice, etc) and in the case of Milk Kefir (Milk from most mammals), the cultures can last basically forever with a bit of care. I still have Milk grains dating back from 2007, when I started that journey into fermentation...
In both cases, but especially with Tibicos, the fermentation process produces a lot of "grains".. Milk Kefir takes a while, eventually it also produce more grains but not as much as Tibicos.
Fermentation is fascinating: for exemple Kombucha and Water Kefir are VERY different , yet , you could ferment sugary tea with tibicos and it will produce something similar to Kombucha .. or most often not

... Sour and drinkable but, there again the resulting "seeds" for the lack of a better words, may not be as strong and long lived as an original Kombucha "Mother"...
Some people have taken to ferment coffee with Kombucha mothers and it seems to work.. Not tasted such , not cared enough to anyway ..
For those who are interested" Fermenting Grape Juice with TIbicos produces a beverage with serious (not measure by me) alcohol content...
Peace.