Crossing the bar

where did saying crossed the bar originate

I've looked at several references including two specifically on sayings with origins in seafaring. No luck yet. The Tennyson poem doesn't explain what it means:

Crossing the Bar
Alfred, Lord Tennyson

Sunset and evening star,
And one clear call for me!
And may there be no moaning of the bar,
When I put out to sea,
But such a tide as moving seems asleep,
Too full for sound and foam,
When that which drew from out the boundless deep
Turns again home.

Twilight and evening bell,
And after that the dark!
And may there be no sadness of farewell,
When I embark;
For tho' from out our bourne of Time and Place
The flood may bear me far,
I hope to see my Pilot face to face
When I have crost the bar.

Here's what one source says:

"Crossing the bar" refers to the death of a mariner. The phrase has its origin in the fact that most rivers and bays develop a sandbar across their entrances, and 'crossing the bar' meant leaving the safety of the harbor for the unknown."
www.usmm.net/poems.html#anchor448654

Whilst Tennyson indicates death, he uses this metaphor without any suggestion of danger.