[piosenniel]

Let's take the riddle apart a little:

O! See the waters break upon the prow,
Their shimmering mists leap up to limn the trees,
That mast-like stand behind its rocky bow -
No sails to move the ship to salty seas.

These lines give the definite image of a land structure shaped like a ship. The waters break against the front (prow) of the ship - their mists rising up to the trees that stand along the top of this land structure like masts. The last line of this stanza indicates it is not found in the sea.

So, we have something shaped like a ship, in fresh water (obviously a river, since the waters break against the front of it.)

Now the final clue:

And let the fires burn at Amon Dîn,
Should shadow breach the way and enter in.

Amon Dîn is the first watchtower north of Minas Anor(Tirith), used to alert Gondor to an invasion of the enemy from the north or east. This geographic structure must be near there and must be important as a defense against invasion across the river from the north and east.

So, we've got an important land mass in a river north of Minas Anor that is shaped like a ship and would be a place that Gondor would want to make sure enemies did not capture and then enter into Gondor.

[ April 08, 2003: Message edited by: piosenniel ]