You must note Hunter that I do not agree with a lot of modern day etymology! That is, I find that they tend to ignore some obvious relationships, or they are unable to actually recognize them, or they merely discard them for the most accepted version, etc.!
Thus, before you get all mired down and end up searching and find out that "hippo" actually seems earily similar to "hippa", etc., and then migrate to "potamus", etc., then perhaps you might want to read this;
The following can be found at "The word detective.com"
Dear Word Detective: A co-worker and I were having a discussion on the word "potable." Where does that word originate, and why isn't it just "drinkable"? His theory is that you put water in pots, hence "potable." I say hooey. -- Matt Meade, via the internet.
I say "hooey" too. In fact, I say "hooey" so often during an average day that one of my dogs now answers to "hooey." Of course, she also answers to "tuna," "telephone" and "lint," so maybe I just need a smarter dog. But "I say hooey" makes a fine all-purpose motto, and as soon as I finish answering your question I'm going to have some "I Say Hooey" t-shirts made up.
In the case of "potable," however, I can only declare a "partial hooey" on your co-worker's theory. Although he isn't exactly right, he's not completely wrong: there is a connection between "potable" and "pot."
It all began way back with Indo-European, the precursor to most modern European languages. The Indo-European root word "po" meant "to drink," and gave us the modern words for "to drink" in French, Russian and Welsh, among other languages. In Latin, "po" produced the verb "potare," also meaning "to drink," and "potare" begat "potabilis" ("drinkable"), which led directly to our modern English "potable," which first appeared around 1572.
Now we'll take a few steps back and explain why your co-worker is not completely crazy. That little root word "po" also produced the Latin noun "potio," meaning "a drink," which eventually gave us the English words "potion" and "poison." A related word ("potus"), also meaning "a drink," survived into medieval Latin, where it came to mean "a drinking cup or vessel." This word "potus" gave us a wide variety of modern English words, among which are "porridge," "putty," and, as you've probably guessed by now, "pot."
So while "potable" does not come directly from "pot," they do share a common (and very prolific) ancestor.
As to why we say "potable" rather than "drinkable," there's no particular reason, aside from tradition and a historic preference among English grammarians for fancy-schmancy Latinate constructions. But there's nothing wrong with saying "drinkable," especially considering that we've actually been saying it since around 1611."
And to the "Word Detective", I also say "hooey!", as well as "phooey!", ahem!
I assert, as my God given right, to propose that "potamus", etc., meaning river,or stream or even "spring", also meant "drinkable!" or "fresh!" This was of course from a time when such water supplies were certainly more drinkable than todays!
Hippos, of course, means the same as "horse" and surprisingly, horses or known to drink from rivers and streams, as were men and women during the ancient past! Boiling water to drink, was still a few centuries distant, depending upon, of course, where one lived and from what place in the stream or river one drank from! Surprisingly most humans develop a resistance to water that would make us all sick, just as horses have the same resistance!
I contend that "potamus" and its various spelling, meant at one time, only "fresh water" as opposed to "salt" or "sulfur" water, etc.! Thus in those days whilst one "could not lead a horse to water", it was certain that a horse could lead a man to "fresh water!"
This is my theory and I'm sticking to it!