What is the moral lesson of "Crossing the Bar" by Tennyson?

The moral lesson of this poem is that we should not fear or mourn death because when we die we are going to meet our "Pilot" -- we are going to meet God.

We see this theme in the second half of the poem more than in the first.  In the second half of the poem, the speaker asks that there should be no sadness of farewell, that people should not mourn his passing.  He says that this is so because he will meet God when he crosses the bar.

The first part of the poem has a different message, but one that is less to do with morals, in my opinion.  In the first part of the poem, the speaker is hoping that his death will be easy -- that the weather will be good for the voyage.