Underwater seismic testing sounds like dynamite going off outside your home, over and over again. The sound carries for thousands of miles, and marine animals can't escape it. Dolphins and whales might beach themselves to get away, or, if they're too close, they could suffer hearing loss or other injuries.
The loud noises can make communication impossible, cutting these social animals off from their families. When faced with this noise, endangered whales stop singing for mates and dolphin mothers can no longer whistle at their babies.
http://act.oceana.org/letter/l-seismic/