Found it!
(despite some forgotten d's and n's...
)
"Blame me not, as one unwarned, for what may then betide; may the power of the Lord of Waters be shown indeed! For in that hope alone have I been willing to guide you, and if it fails then more surely shall we die than by all the perils of wild and winter."
Voronwë speaking to Tuor about the peril of approaching the first gate .