When you consider the life-giving nature of water (drinking, irrigation), its spiritual connections (baptism, ritual cleansing) and the hugely distructive ability it has when unleashed (flooding, erosion, drowning), is it any wonder great power is attached to the idea? 

Goldberry, with control over the element of water, would be considered an immensely powerful figure in any mythology, but benevolent. The Black Riders/Barrow wights are clearly not. The Brandywine River that killed Frodo's parents also saved him from the Black Riders. These dualities exist throughout the book when it comes to natural forces. Tolkien never denies or hides that every power has its perils.

"...their hearts, wounded with sweet words, overflowed, and their joy was like swords, and they passed in thought out to regions where pain and delight flow together and tears are the very wine of blessedness."
--the words of the Professor