...it was a dream
Copy yousr stroked object, and paste in front
Then scale 70% in transform palette ( set the reference point appropriately, click the chain so both width and height scael, add * .7 after width or height, then hit enter)
... or a nightmare.
It is achievable, though, with more or less effort, depending on the shape and the desired way of filling, including:
B) Pie chart/star/pin/stripe style, etc,
C) Straight/rounded/wavy/zigzag, etc,
0.7 for both W and H would yield 49%.
70% at the centre or somewhere inside (just one of the vast number of options, which might put some of the filled area outside the path in some cases) would require 0.83666 for both W and H.
I want to fill the object 70%
What does that mean?
You want a solid fit that spans 70% of the object's height? Width? From the top, bottom, middle?
Fill it with a uniform pattern that covers 70% of its area, like a 70% tint screen?
There's a kazillion ways to guess at what you're trying to describe. Show a sketch or something.
To me it looks about 54% water and 16% filled with fish.