In his famous 1984 speech, Chief Seattle is grieved to speak of how the tribal people of America who ‘once filled this vast continent as stars fill the firmament’ has come to the edge of premature doom.

To clarify his views Seattle has used several similes in his speech. The two comparisons cited in the above question are important similes where the speaker compares and contrasts the state of the native people in the past and the present.

“There was a time when our people covered the land as the waves of a wind-ruffled sea cover its shell-paved floor, but that time long since passed away with the greatness of tribes that are now but a mournful memory.”

The speaker here compares the past situation of the tribals when they were in their glorious phase to the waves of a wind-ruffled sea. He means to express how lively and energetic their people were then.
Again, Seattle says:

“Our people are ebbing away like a rapidly receding tide that will never return.”

Here he compares the present situation of his tribe, at the time of the speech, to a rapidly receding tide, that is going back from the shore, losing its might and with no hope to return again. This symbolizes the decay of the native people.