Need to read/send both values of a button push
Change

else if (isButtonPressed==0 &amp;&amp; flag==0)   
    Blynk.notify("Alert : Water level has risen");


to

else if ((isButtonPressed==0) &amp;&amp; (flag==1))
{
    Blynk.notify("Alert : Water level has risen");
}


Otherwise, when flag == 1, it will never revert back to 0.
Also it's best to add brackets, and use parenthesis.

Also, change the variable name flag to something more meaningfull.