Answer: not likely
Explanation:
The scuba instructor stated that seeing a shark was improbable, meaning that there´s little chance of that happening, but it could. He even asks people not to panic if it happens, which reinforces the idea that a shark could appear, despite the little probability of that event occurring.
"Not likely" refers exactly to that kind of situation, as it means that something probably will not happen, but there´s still a chance that it will.
