How much water is left?
Pyth - 17 46 45 52 * .85 - 35 = 9.2 bytes
Filters the input (With the new # filter meta-op!) for a line with ~ in it, then indexes that to the input, and then divides that by the length of the input. If there are none with ~, it errors, and triggers the except clause of .x and prints the string.
.x+*100-1cxK.zh@#\~KlK\%.&quot; u(C$éáPãbÉãç*îÂe[W

Try it online here.