SEABROOK, Hemel Hempstead, circa 1800
He wonders if there could be a link with his ancestor Henry William Seabrook (b.c.1807) & Sarah White (b.c. 1805 Norfolk?) who married 1828 in Southwark Surrey, who had a daughter Sarah Caroline Seabrook (bapt St Lukes Old St, London in 1831) and immigrated to Tasmania, Australia, ex London (June 1832) with Sarah's brother Thomas White ( b.c. 1807 Norfolk) & wife (Sarah Coaton b. London to Marmaduke Coaton & married c.1827 London) & arrived Hobart 12 Nov 1832 on the " Thomas Laurie" as assisted carpenter. A George White possibly a relative left London for Tasmania on same date. (also an assisted immigrant -carpenter with wife & child George & Ellen). Henry William Seabrook & T White were both apprentices (carpenters?) to Sir William Cubitt, builders of London, & were prominent builders in Hobart, Tasmania (1840's-1870's).
My ancestor George Smith Seabrook b. 1838 Hobart & also a builder, was second son (first son was Henry William b. Hobart 1836) & I have a strong gut feel that George Smith Seabrook's grandmother on father's side was a nee "SMITH." It is noted that these names (George, William, John & James) were used often in the Tasmanian Seabrook descendants.
The overall distribution of the Seabrook surname clearly suggests that your Henry William Seabrook could have come from West Hertfordshire or the adjacent parts of Bedfordshire and Buckinghamshire. The pattern of given name are suggestive (see The Inheritance of Single Christian Names) and may be relevant that a 2 year old William Henry Seabrook was buried at St Mary's, Hemel Hempstead in 1848 (National Burial Index). It is also interesting that Susannah Smith (baptised 1777 in Hemel Hempstead) had a brother George Smith (baptised 1774) so if your suggestion is correct your George Smith Seabrook could have a great uncle George Smith. However common family names can lead to confusing situations where genealogical errors are easy without good records (see Right Name, Wrong Body).
I assume your Henry William Seabrook is the William Henry Seabrook who died in Tasmania in 1883 aged 76 and it is not impossible that he was the William Seabrook baptised in 1802. (I don't know what the conditions of assisted passages were but I am sure it was designed to encourage young men to emigrate. If you had to be no older than 25 he had an excellent reason for falsifying his age.) However we need to be cautious as a 5 year old William Seabrook was buried at St Mary's, Hemel Hempstead, in January 1805 (National Burial Index). A check of the 1881 census shows one William Seabrook born in Hertfordshire in 1802 was still living - but he was born and still living at Long Marston so is unlikely to be the one baptised in Hemel Hempstead.
Basically there is no firm evidence that your Henry William Seabrook was born in Hertfordshire, much less that he was the son of George and Susannah. If you have not yet checked originals of the apprentice records (which may include the father's name) I suggest do so now. You should also check the Hemel Hempstead registers - as it may even be that the IGI index is in error. If you want to find out whether George and Susannah left Hemel Hempstead between 1804 and about 1807 you may be able to find some clues from the records from the parish chest, such as the overseers and poor rate account books, which are orderable on microfilm from your local LDS Family History Centre (address on familysearch).
If you can add to the information given above tell me.