time delay between signals = 0.012 seconds
speed of sound in water= 1482 m/s (from http://library.thinkquest.org/19537/Physics4.html)

assuming that the source and the two boats are colinear, the distance between the boats is:

Distance = Speed * Time
	 = 1482 m/s * 0.012 seconds

Distance = 17.8 meters