So I beat the game, main quest, factions, all of it. I download dragonborn and I get the boats to take me from port to port but nothing else. I have been in riften sence 4pm ( actual) but no sign of the cultists. Whats goin on?

Same thing is happening to me, I really hope someone can answer this.

Try fast traveling somewhere. That should shake them loose.

For me, they appeared where a random wilderness encounter usually occurs (i.e. on the road between the Battle-Born Farm and the Whitewatch Tower in Whiterun Hold).

Fast traveling to towns *and going inside them* is your best bet. If not, delete Dragonborn from your HDD, clear your game and storage cache, redownload Dragonborn and try again.

Also, you must have recieved the first quess from the greybeards. The one where you get the horn

Also, be sure to have downloaded the latest update, as well.
I wouldn't be attacked by cultists, or have the new boat, until I had done so. Simple solution, but an easy mistake to overlook.

Also make sure when you first start the xbox, and it says to press start, that the boss comes up and says loading add-ons