Final line = 808.

975949
634536
182018

Process/pattern. The products of each pair of numbers in the first line equal the numbers in the second line and so forth
First line: 975949 = 9*7, 5*9, 4*9=  second line 63, 45, 36 respectively
Second line: 634536 = 6*3, 4*5, 3*6 = third line: 18, 20, 18 respectively
Third line: 182018 = 1*8, 2*0, 1*8 = final line: 808

8.33 lbs of water evaporated

(89-x) = (100-x)*.88
89-x = 88-.88x
1-x = -.88x
1=.12x
x=8.33

Where:
89-x = water loss
100-x = watermelon loss
.88 = new ratio

100-8.33=91.67 lbs (new watermelon weight)
89-8.33= 80.67 lbs (new water weight)

80.67/91.67 = 88%