Platt is the dialect spoken in the North of Germany. Plattdeutsch (Low German, Low Saxon) is regarded as a language of its own and shares many features with Dutch. As many local languages, however, the 'real' Plattdeutsch is rarely spoken any more, yet, the remainders show in the dialect(s) of Northern Germany. (NB: "Northern German Platt" is actually an oversimplification, as each region have their own "Platt".)
"Wasser" was "Wasser" also before 1996 ("ss" after short vowel followed by a vowel). Before 1996 "ß" was used after short vowels, if it was the last character of a word or if it was followed by a consonant. After 1996 these were changed to "ss" (only after short vowels!).
Some examples to illustrate this:
old: der Fluß, new: der Fluss (the river, short vowel "u")
old: der Fuß, new: der Fuß (the foot, long vowel "u")
old: Wasser, new: Wasser (water, short vowel "a" followed by a vowel, not by a consonant
old: ich muß, du mußt, er muß, wir müssen, ihr müßt, sie müssen; new: ich muss, du musst, er muss, wir müssen, ihr müsst, sie müssen (short vowel "u" for all conjugations)
old: das Schloß, die Schlösser; new: das Schloss, die Schlösser (castle[s] or lock[s])
Are you asking this question because DuoLingo would not accept "That's water" as a translation of "Das ist Wasser"? If so, don't worry: "That's water" is a fine translation of "Das ist Wasser" and should actually be accepted, in my opinion.
On the other hand, if you are asking because you are interested in how people would actually pronounce, contract or slur the words "Das ist", the answer is "Das's" or "Dassis" :-)) Contrary to the English contraction form "that's", however, no one would ever write "das's" or "dassis", not even in an informal SMS, unless they try to be funny.
Both "dass" and "das" exist in German, but the pronoun (with the possible meanings of this, the, that, it) is written with one s. The word "dass" with double-s is a conjunction (meaning "that" in a sentence like "I know that you are learning German" = ich weiß, dass du Deutsch lernst).
In the 1990's, German spelling rules were reviewed and revised for the purpose of better consistency ("neue deutsche Rechtschreibung"). Several updates and amendments, mostly on details, have be issued since. Apart from this relatively recent change, my feeling is that the German language changes as much or fast as any other living language in Europe. You are certainly right that globalization and Americanization takes a significant toll on idiosyncrasies, idioms, local peculiarities.