Thanks Imber, I looked up the Merchant Navy Lists on FindMyPast for two people:
Robert Holland, born Shrewsbury Shropshire in 1861
George Donald Perry, born West Cowes, Isle of Wight in 1858
I haven't been able to match them to entries in the Merchant Navy Lists using year of birth or place of birth though for George Perry there are a number of entries without these facts which I have yet to explore.
Because they probably arrived in South Australia by being discharged from (or walking away from) their respective ships, there is no immigration record for them, so their ships and year of arrival are unknown. George some time before 1881, Robert some time before 1891. George worked on yachts and tugboats at Port Adelaide about and after 1881 and before that was possibly on at least one voyage on wheat or wool clippers. Robert also lived at Port Adelaide, went away from his family in Australia on long voyages, including some to the United States.
Ta.