Answer: It suggests that the mate has been a seaman a long time and his skin is aged.
Explanation:
"My men grow mutinous day by day;
My men grow ghastly wan and weak."
''The stout mate thought of home; a spray
Of salt wave washed his swarthy cheek.''
In this way he is describing a man who has been sailing for so long and that his skin aged. The writer is suggesting how the seaman spent his life sailing and he aged while he was doing that.
''They sailed and sailed, as winds might blow,
Until at last the blanched mate said:
"Why, now not even God would know
Should I and all my men fall dead.
These very winds forget their way,
For God from these dead seas is gone.
Now speak, brave Admiral, speak and say" --
He said, "Sail on! sail on! and on!"
Through the poem he is using ''Sail on! Sail on!'' to suggest on the most important thing for the seaman's and how they was doing that every day, night and day. They did not have a brake or holiday, they were sailing for a long period of their lives.