An anchored fishing boat used sonar to work out how deep the sea is. it sends out a ping of sound and detects the echo 2 seconds later
how deep is the water? ?

This is easy if you know the speed of sound through water.  That speed depends
on the temperature of the water, but in the range of temperatures in which you'd
want to be out fishing, it's around 1,480 meters per second.

So, in 2 seconds, sound travels (2 x 1,480) = 2,960 meters, in water.

But the sound has to go down and back up before the boat hears it, so the
actual depth of the water is only half of that distance.

1/2 of 2,960 meters = 1,480 meters deep