Why is "The water is blue" not acceptable?
There is no "das" before "wasser"
why did Duo say I had a typo when I used the sharp S letter or eszett? I thought that could be used anywhere a double s was used?
I think it can only be used when the preceding vowel is long. Native speakers, please correct me if I'm wrong.
Yes, since 1996 the spelling rule has been that ß is used after long vowels and diphthongs and ss is restricted to use after short vowels.
They are not interchangeable.