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I think the Ann you are looking for may be Ann Rumell/Rummell who married Thomas Roberts in 1784 (probably after the birth of Thomas Rumell Feb 1784) which would explain why he doesn't have his father's name) with a Faculty Office Marriage Licence. This allows a discrete marriage with no publicity. I don't know if she married Thomas's master as there are several Thames watermen called Thomas Roberts. The most likely candidate is Thomas Roberts born about 1763 (could be older), apprentice waterman from 1778, master Roberts at Rothehithe. I've seen a copy of the will of Thomas Roberts who died 1789. He was based in Rotherhithe and owned boats. He had five children including a son Thomas Roberts. For more info see my site
https://sites.google.com/site/rummellfamily/