Malbim: Na'aman should not think that Elisha cured him via natural means or an incantation. He wanted to show that the Navi's decree works.

Malbim: Cold water aggravates Tzara'as, which comes from white Lechah ¹ . Na'aman should know that Elisha cures an affliction via an affliction - this increases the level of the miracle.