There are no environmental benefits in the short term, other than the earth was able to perform an act that is as natural as any other. If people were not around, you could make the case that it is a cleansing process that occurs naturally from time to time, like forest fires.
The disadvantages are that all the man made waste, chemicals, and debris are washed back out to sea when the waters recede. This causes many dangers for ALL the indigenous sea life.
Also, many types of fish that spawn in fresh water will see immediate impacts in their numbers as the tsunami will have wiped out their eggs from the fresh water beds that normally protect the eggs, hatchlings, and fry