Yes, I'm curious about this one as well. Not a name I'd come across before, and so far as I know the only place or families it appears "on the boats" is Wombourne, in the Bowaters and Northwoods, which tends to suggest there may be another, earlier connection between the families. My GGrandmother was also Tryphena, christened for her grandmother in Brierley Hill, but the name was not used again in subsequent generations (grateful for that) - on the other hand it helped with tracing the family because it is very unusual.
Now, on the subject of the Underhills - I will ask you to advise which were children of the first marriage, and which the second.
I have Stephen Underhill's parents as Edward and Mary, and he was baptised 29.2.1799.
First wife : Sarah Bowater
Second wife : Hannah Northwood.
Children : Mary 1822, Edward 1825, Sarah 1828, Tryphena 1930, Ann 1833, Louisa 1836, Stephen 1839
Stephen died young, and I have no idea what happened to the other children, and would love to know.
After Stephen Underhill died, Hannah remarried 31/10/1842 in Wolverhampton to James Wright, also a boatman. I found them on the 1851 census at Wombourne, Hannah aged 50 and James 45.
Just for completeness, I also found a sister to Stephen Underhill, Rhoda Underhill, baptised 2/4/1804. She married Thomas Gould on 25/12/1830 at St Peter's in Wolverhampton.
HTH