I live in Southend, and I'm fairly sure there's not - I would certainly have had reason to buy one on several occasions. Tickets from Southend to the ends of the other coastal branch lines in Essex don't come cheap! In fact sometimes a ticket from London can be cheaper..
The nearest thing I'm aware of, doesn't seem to be available anymore (at least, I can't find any reference to it, where I'd expect to, where I'm sure it used to be mentioned, on the Essex County Council website). There was, recently, a ticket valid on all buses in the (administrative) county (and thus not Southend or Thurrock) which was also valid on trains on "branch lines" (I think defined as Wickford-Southminster, Witham-Braintree, Marks Tey-Chappel & Wakes Colne, Colchester/Colchester Town - Walton-on-the-Naze/Clacton, Manningtree-Harwich Town).
Unfortunately I can't find any current references to it, so my conclusion would be it's been withdrawn.
(The Essex Sunday Saver, valid on all buses in the county on Sundays, at £2.80, however, does still exist, and is remarkable value, even if buses are scarcer than they used to be)