Most medium and major river outfalls (perhaps except where there is a delta) have a bar - sand or shingle. The Doom Bar @ Padstow is one of the best known (especially as the Rock brewery named a beer after it!). My nearest one is the Bideford Bar (or Barnstaple bar depending on which town you come from).

From the shipping yards on the Torridge side,
O'er the Bideford Bar to the ocean wide,
Sailed Frobisher, aye, and Grenville too,
In ships of fame with the world to view.

The 'moaning' of a bar is the sound of the tide across bar, particularly of shingle bars - a danger sign that the water is low over the bar, hence 'may there be no moaning of the bar, when I put out to sea'. DaveRo is spot on above although I don't find the metaphor peculiar at all.

The poem is usually regarded as a passage of death poem - hence its use in many funeral services (including that of shantyman John Short). At the last count, I am aware of 100 different settings - from Ralph Vaughan Williams to John Philip Sousa - google it and you'll see. Rani Arbo's setting came into Britain with Jeff Warner.

I've never thought of 'one clear call for me' as having other meanings - other than a clean death in a strong faith. If only I believed in God I'd chose the poem/setting for my funeral.