It's crazy complicated. But:
If you are concerned with heat-loss due to evaporation, then reducing the exposed surface area will help. The rate of evaporation is proportional to surface area.
If you are likewise concerned with maximizing heat-gain from the sun, then that too is proportional to surface area.
Finally, heat loss due to convention is also proportional to surface area.
So everything you care about is strongly affected by surface area, thus maximizing the amount of water that is covered is a good goal.
It's impossible to estimate how important this will all be as far as actual water and heat loss/gain - you haven't given enough information in your question to judge that.
The unanswered question is how much heat and water loss your pool is experiencing, and what is preventing it from losing more. You might alter the rate of cooling/water-loss without actually affecting the ultimate final temperature and total water-loss.
Example: If I have a leaky bucket that I fill up with water once, it doesn't mater how quickly the bucket drains because once it's empty there isn't any more water to lose. If I am constantly topping up the bucket, then fixing those leaks is more important.
If I were you, I'd conduct an experiment. Leave the cover totally off for a few days and measure whatever you care about (water level, temperature, pool-heater-energy-consumption). Then do the same with the cover on, and perhaps again with the cover 1/2 on. That should give you enough information to make a gut decision on how much the pool cover is helping.
Answered by OP in comments:
I did it! There is something like a little groove at the side - at the connection between black and white parts - wherein I slotted a screwdriver, twisted it, and the light cover pulled off. Eureka! – Lukman Dec 17 '16 at 12:16
Best Answer
You may want to consider just applying some of that opaquing film to the window glass that just goes part way up. You wouldn't get your plant shelf but this would be an easy solution to the people looking in.
Since you live in an apartment your range of options of that you can do are limited unless this is a unit that you actually own. And even in the case that you do own you would not want to go so far as modifying things in a way that they are not reversible without damage.
One scheme that should work would be to look into using a sheet of the dense pink or blue rigid foam insulating material that is cut to length so that it fits across the window opening with a push-in friction fit. This material comes in large panel sizes (in USA ~4 feet by ~8 feet) so you could achieve the size of covering that you need.
For the shelf it may very well be possible to cut a narrow strip of the same material that fits into the opening and sits flat on the upper edge of the material that covers the window from below. It could bonded or pinned to the lower panel as well.
To improve the look you can try spray painting the foam to a color that coordinates with your room.
Do note that I am not suggesting that you use the lower cost white crumbly foam panels.