This is a really tough one.
I found your tree on Ancestry and I see that your great grandfather Charles Jones b 1882 in Melbourne (adopted surname Wellman) had a very sad early life, losing his Manx mother Elizabeth in 1884, and being abandoned by his father.
From the few scraps of information available (you say that her marriage cert gives her parents’ names as John Cowell and Margaret Watson ---- I can’t see a copy of the marriage cert on your tree) we’d expect to find a baptism on familysearch to a John and Margaret Cowell, or an illegitimate baptism to a Margaret Watson. We’d also expect to find census entries for her in 1861 and 1871 on the Isle of Man or on the mainland.
Nothing I’ve found seems to fit, so I started looking at Watterson rather than Watson, and at Cowle as well as Cowell (although Cowle and Cowell weren’t strictly speaking interchangeable, they were sometimes confused because they sounded similar).
I’ve come up with a possible candidate who also had a sad early life, being orphaned age 11. Her family’s surname is spelt both as Cowle and Cowell in various censuses and in the baptism and marriage registers.
17 March 1850 Arbory --- Elizabeth daughter of Norris Cowle and Margaret Watterson, labourer, abode Malew.
2 Jan 1848 Arbory --- Anne dau of Norris Cowle and Margaret Watterson, labourer of Malew.
18 Aug 1844 Marown --- Robert son of Norris Cowell and Margt Watterson
26 Jan 1840 Patrick --- Thomas son of Norris Cowle and Margaret Waterson.
1 Aug 1852 Arbory --- William son of Norris Cowle and Margaret Watterson, labourer Malew
4 Sept 1842 Marown --- Margaret dau of Norris Cowell and Margt Watterson.
7 May 1863 Kk Lonan --- Marriage --- William Cowin married Margaret Cowle, spinster/residence Begode, Onchan/father John Norris Cowle, labourer.
4 Nov 1855 Onchan --- Catherine dau of Norris Cowell and Margaret, labourer of Little Mill.
24 March 1880 Malew --- Marriage ---William Gale married Catherine Cowell/24/spinster/Ballagilbert/father Norris Cowell, labourer.
Ballkelly ? Cottage, Marown
Norris Cowle 27 (Ancestry has Louis)
Norris Cowell 35 lead mine lab b Marown
Margaret 35 b Malew
Thomas 11 b Patrick
Margaret 8 b Marown
Robert 6 b Marown
Ann 3 b Malew
Elizabeth 1 b Malew
Margaret Cowell widow 46 housekeeper
William 8 b Onchan
Catherine 5 b Onchan
others all b Arbory
9 Oct 1861 Onchan --- Margaret Cowell age 44 of Little Mill.
You need to suspend your disbelief for this next one:
Leadenhall St, Everton
Bertha Cowell unmarried 19 general servant b Isle of Man (Ancestry has transcribed as Hertha)
I’ve looked at lots of possibilities for this to be a different person who was actually called Bertha but can’t find her in any other census. Nor can I find a Bertha b IOM who fits the bill.
Perhaps Betsey’s name was badly written and then misread for the neat copy.
So could this be the 23 yr old Eliz Cowell who sailed to Melbourne from Liverpool in 1875, who married Robert Charles Jones in Victoria in 1880 ? I think it might be.