Don't understand the answer to this question!?

I think you misinterpreted the question. Basically, what it's saying is that a sound is generated somewhere and travels through the water to the fish and also through the air to the fisherman in the boat. Because sound travels faster in water than air, the fish will hear the noise before the fisherman. You just need to figure out the difference in the time it takes the sound to travel 6.76km through the water vs. the time it takes for it to travel 6.76 km through the air.