What the Bible says about Freshwater as Metaphor
(From Forerunner Commentary)
The apostle reveals here the nature of "the bit and the rudder" that control the horse and the ship. It is spiritual wisdom and understanding that descend upon us directly from God above that empowers us to keep our tongue in check. As earthly men, we have no means or strength to control this wild beast that resides in our tongue and expresses itself in our words. But as children of God, endowed with the power of His spirit and wisdom of His character, we can gain control over this member within. Not only that, we can use it in service to His will and purpose. If we examine our words and run them carefully through the filter of the fruit of the Spirit, we stand a far better chance of releasing the fresh waters that give life and restraining the salt waters that cause destruction.