Table schema:
CREATE TABLE table_name_90 (silver INTEGER, gold VARCHAR, sport VARCHAR, bronze VARCHAR, total VARCHAR)

Question: What is the smallest number of silvers associated with bronzes under 20, totals of 1, golds of 0, and in Water Polo?
SQL: SELECT MIN(silver) FROM table_name_90 WHERE bronze < 20 AND total = 1 AND sport = "water polo" AND gold < 0