Get a copy of River-Horse by William Least Heat Moon (who wrote the bestseller Blue Highways years ago.) When he's not going off on tangents he tells the story of crossing the country by small boat, east to west, portaging by trailer when he has to.
The Mississippi delta is not just a few divergent branches and then a pop into the sea - take a good look at some charts and Google maps! And there's some serious commercial shipping there, so MAKE SURE you can get out of the way!
Also, if you're planning to go "to the ocean and back" but not out on the ocean, a sailboat is probably not your best choice for the river, especially coming back up! These days it's amazing how much powerboat you can get for a few thousand, and on the return trip, going against the current for 1200 miles, you're going to need a little power. I'm a sailboat-er, for sure, but on the river, since you're not going to be doing any appreciable sailing anyway, a powerboat would have much more living space and be more comfortable.
PS - Originally I come from Pittsburgh, up the Allegheny from the Ohio, so I've been on that water. There aren't nearly as many barges traveling there as I remember (is "Rocket" still rolling?), but damn, I'd really, really hate trying to both keep my keel out of the slurpy muddy banks and keep my "sailboats have right of way" with a barge!
Good luck, and remember, an adventure is never, ever what one expects: that's the rule!