Well it depends on a number of factors. open seawater is about 3.5% salt(closer to 3.2% on a tropical coast. So, for each 1000 litres of water they drop they would be dropping about 35kg of salt. Then it's a question of what area it is dropped over and how soon it rains and the runoff to dilute the salt. In addition, it also depends on the flora(and fauna) as some are more salt tolerant than others. At minimum it would modify the ecology however the fire would have done that too.
