Albert Barnes Commentary Exodus 15:25

"An he cried unto Jehovah; And Jehovah showed him a tree, and he cast it into the waters, and the waters were made sweet. There he made for them a statute and an ordinance, and there he proved them;" — Exodus 15:25 (ASV)

A tree ... - The statement points to a natural agency, but the result was manifestly supernatural.

He made ... - The Lord then set before them the fundamental principle of implicit trust, to be shown by obedience. The healing of the water was a symbol of deliverance from physical and spiritual evils.