Searching Google with "Trinity High-Water Mark" in quotes (and adding London or Thames) returns a bunch of Google Books results from the 19th century. My favorite is this transcript of the Reports from Committees for the Great Britain House of Commons:
5838 . Mr. Forsyth.] Is the Trinity a datum level ? -- The Trinity high-water mark is the datum.
5839 . Where is it, as a fact? -- Twelve feet 6 inches above Ordnance datum. Trinity high-water mark is one of the heights which is commonly taken by water engineers as their datum line. The Ordnance datum is the datum taken by railway engineers, Trinity high-water mark being 12 feet 6 inches higher than the Ordnance datum, the latter being supposed to be the mean-tide level.
Apparently they had a map at the time, too, which might be on the record somewhere?