After much experience and experimentation, trial and error, and deciphering of the wording on Bulbapedia, I believe I may finally be able to answer my own question. The Great Marsh is separated into 6 individual areas. It appears that there is a separate daily binocular Pokémon for each of these 6 areas, and are completely independent of each other, so it is possible for multiple areas to have the same daily binocular Pokémon.
Quote from Bulbapedia:
These Pokémon will appear randomly in one of the six areas; some may not appear for a while, while others may appear several days in a row or in multiple areas on the same day.
So to answer this in the most simple way would be to state that UP TO 6 separate Pokémon can appear throughout the Great Marsh. It also appears that pre-national dex Pokémon continue to appear along with post-national dex Pokémon once the national Pokedex has been obtained.
Of course this is all based on my own personal experiences, so if any one could confirm similar/identical experiences, that would be very helpful.