He was quite likely crew. Certainly with at least two brothers as master mariners his seamanship was likely better than average and with apparently ships going from NB to Australia with some frequency, the opportunity would exist.
As for sources, I begin basically with census returns:http://www.bac-lac.gc.ca/eng/census/Pages/census.aspxhttp://automatedgenealogy.com/census52/http://automatedgenealogy.com/censusnb51/
Then I go to various databases at the Provincial Archives of NB:http://archives.gnb.ca/Archives/?culture=en-CA
After that there are a number of compilations I have found, some still existing.
I know that the Chief of Police, John R. Marshall had a brother in New Zealand. I feel confident that any David Sim Marshall of the period must be either a son of David Sim Marshall, Sr. or of his (apparently only) brother, John Marshall, Jr. I can find John, Jr.'s son, David Sim Marshall, so your David must be the son of David, Sr. and the brother of John R. Not conclusive but, with other factors, very good circumstantial evidence that I find convincing; especially, if as you assert, your David was from NB. Most people only know of NB that there is a city 'St. John's'. Actually St. John's is in Newfoundland and the City officially renamed itself Saint John although in early instances, St. John is legitimate. So, saying someone (in Minnesota, say) was from 'St. John's' is indicative, but not compelling evidence that they ever saw that city.
But the key is interpretation which requires a certain familiarity. For example at:http://automatedgenealogy.com/censusnb51/View.jsp?id=5411
people often think that 'Native' means 'Native American' and assume that they are First Nations. Actually, it just means that they are supposedly born in NB; and even that can be incorrect. Otherwise, it helps, for example, to know that the Wightman's were Loyalists from Rhode Island, descendants of Edward Wightman, last person burnt at the stake in England for heresy (and he was guilty) and major proponents of the Baptist Church in North America.
I recommend trying to contact me through clicking on my name and contacting me directly with an e-mail. I can't as I am not a subscriber. Alternatively, you can post an e-mail address with 'masking' as in johndoe'at'peabody'dot'org or something similar. Never give your e-mail address in a post as is, nor dates-of-birth of living persons.
It is now 12-pages with some surprises. It seems that one of your ancestors, Rebecca Upton (David, Sr.'s mother-in-law), married at least three times, something that seems to have been overlooked. It seems that the grantee, David Simpson, in Queens Co., NB, was actually David, Sr.'s grandfather, David Sims. One of David Sim's children, apparently David Sim Marshall, Sr.'s mother, was:
"According to, “Records of the First Presbyterian Church of the City of New York.—Births and Baptisms”, ‘New York Genealogical and Biographical Record’, Vol. X, No. 3, July 1879, NYG&B Society, New York City, pp. 127-133, on p. 130, Janet (Jennet?) Sim, daughter of David Sim and of Jane Nicoll, was born in New York City 21JUL1774 and baptized 28JUL1774."
Stuff like that. Actually, I'll try creating a PDF file and attaching it. Nope, I am not permitted to do that.