Aswimming pool is 1.4 m deep and 12 m long. is it possible for you to dive to the very bottom of the pool so people standing on the deck at the end of the pool do not see you?

So this is smaller than the length of the swimming pool and hence we can say that diver will not be visible to the person sitting at deck

Explanation:

If the diver is at the bottom of the swimming pool then we will say that the nobody on the deck will able to see if light coming from the diver will have total internal reflection

so we will have

here we will have

for water

now we have

now we have

now let say the radial distance from the position of diver is"r"

so we have

So this is smaller than the length of the swimming pool and hence we can say that diver will not be visible to the person sitting at deck