Well, my first thought is, that the fish is too fast.
It will be gone as soon as the stone hits the surface.
And, the speed will also increase ( slow down) with the impact... And the fish will have enough time to be gone..
And how to aim... One must relay on the good luck..
No... I can not see any advantange to fishing with a sling......
Unlesss......, you wait until the fish jumps above the surface.. Some fishes do so...
But then you must be fast and very accurate...
And if you are so lucky to get a fish, you must probably swim out there to get it... Before the sharks come

I´m sure there is more better ways to fishing then use a sling

Ulrica