Wales use sound waves to *see* obstacles, from rock formations to prey. An obstacle will reflect the sound wave back, which tells them something's ahead. You can find reports on the connection between disoriented whales and underwater *sound pollution* due to vessels.
Scientists can use underwater sound techniques for the same purpose. But there are many other reasons you'd want to use them. It can tell you about the properties of the water, and of the materials it reflects off of.