water as a symbol of good

[Tuor of Gondolin]

The genesis for this thread is twofold:
1) My bemusement at many (including, in one of his Letters, JRRT) questioning
why the nazgul were "put-off" by crossing rivers
2)an article in the book by the people behind TheOneRing.net entitled "More
Peoples Guide to J. R. R. Tolkien" by Erica Challis.

As for the former, it seems to me quite possible that it was the power of Ulmo
that was a deterrent to Middle-earth bad guys venturing over, on, in water
bodies, not just in the Third Age, but overall. Recall that Cirdan, even in Beleriand,
never had to deal with threats from the sea.

And the article "Secret Messengers", by Erica Challis' non de plume Tehanu,
generally mentions three forces of good (light, air, and water) but focuses on
water, observing that from the beginning of the awakening of men it was Ulmo,
through messages sent through the sound of water that stirred them. And she
perceptively remarks on an event I've always found most interesting in LOTR:

...when Sam and Frodo are alone in Mordor, darkness oppresses them and thirst torments them. Sam wishes Galadriel could hear them now-he'd petition her for a bit of light and some water, and that would be better than any jewels. Soon after, they see the wind rise up and turn back Sauron's pall of darkness, and not long after, they come across a small trickle of precious water. It's like a prayer answered. To Sam, maybe Galadriel is like a 'higher power' as he can know or understand. But in Tolkien's mind, maybe Sam's power is heard by Manwe himself, who foreknew and sent down a sweetly falling rain where it would reach the hobbits, forelorn in the Dark Land.

And while the ocean(s) as such are only fully utilized by the Numenoreans,
rivers (above all Sirien and Anduin) play key roles in the topography and events
of Middle-earth. Two examples: the Shire being (effectively) an island, which,
when Sam crosses the Brandywine, feels as though he is leaving his world
behind. And Elrond using water to thwart the nazgul at the Fords of Bruinen.
There are of course, numerous other examples of the beneficient effect of water,
from Tom Bombadil and Goldberry's realm to Lorien, as well as the negative
effects of "polluting" water from Lake Ivrin to the Sea of Nurnen.

To what extent, then, is this apparent presence of water as a power for good an
indication that the Valar (and specifically Ulmo) have not abandoned
Middle-earth to the extent it may seem in the Third Age? And any other
comments on this curious primacy of water over other factors as a force for good
in Middle-earth?