What power is Elrond referring to?

[Gurthang]

As far as the water goes, it did have a power; although not necessarily an 'elvish' power.

From Ainulindalë:

…And they observed the winds and the air, and the matters of which Arda was made, of iron and stone and silver and gold and many substances: but of all these water they most greatly praised. And it is said by the Eldar that in water there lives yet the echo of the Music of the Ainur [emphasis mine] more than in any substance else that is in this Earth…