The decision was made last night after tests showed water quality did not meet the required threshold following rain on July 31 and August 1.
The mixed relay race is scheduled for Monday.
"We are expecting an improvement on the conditions in the next hours, but not to a level to which the swim familiarization planned for tomorrow can take place," organisers said in a statement.
"In view of these conditions, we have decided to cancel the swim familiarisation tonight to avoid late communication to athletes."
Pollution in the Seine after heavy rains had caused the men's race to be postponed for a day, after swimming practice sessions were cancelled two days in a row.
--Reuters--