[Tuor in Gondolin]

I have to disagree with the comment above:

Interesting thoughts young Sharku, but Ulmo is really the King of the Seas. The Lord of Waters is a misleading term. Yes, I am known as the Lord of Waters, but salt water only! Ulmo has no influence on such matters as Ent draughts, and waterfalls, or water lillies.

Since in the Silmarillion:

For all seas, lakes, rivers, fountains and springs are in his
government; so that the elves say that the spirit of Ulmo runs in all
the veins of the world.

And the continued influence of Ulmo in Middle-earth waters seem
to me the reason that Sauron and his forces feared water (even the
nazgul). Which leads to the perplexing situation where JRRT said
somwhere (in Letters?) he didn't have a good reason why the bad guys
didn't like water. Ulmo's lingering presence is certainly a believable
cause, as it would be an enabling factor in Elrond controlling surrounding
rivers to protect Rivendell.