I was told the following (by an American relative!):
"St Nicholas’ church in Liverpool was the ‘Gretna Green’ of the North West. Many marriages were performed for people from as far away as North Wales, and regularly from Cheshire. The church did not ask too many questions as to whether the parents of the couple approved of the wedding or whether the bride was pregnant. Therefore it was relatively easy to marry at this church."
My Great Great Grandparents were also married here, in 1840, and they travelled from Chester to do so (they were both underage, and they were cousins - possibly a union which was frowned upon). He was in the brewery trade, not a seafarer.
Another pair of Great Great Grandparents travelled from Heswall to the same church, in 1842, the groom giving a Liverpool address, which I have been told was an "address of convenience" for the marriage, as they moved back to Pensby afterwards. He was an Agricultural Labourer, no seafaring influence there!