TRIGGER WARNING for violence at embedded link
the photos at the source show theon getting completely wrecked by the Ironborn. Also in the pics, director Jeremy Podeswa is there which most likely means that this scene happens in Season 7 episode 1.
The reddit leak said:
-Euron's fleet destroys Yara's and he takes her prisoner. Theon abandons her, jumps overboard and is rescued by some Iron Born.The Iron Born lose any remaining respect they had left for Theon.
-Euron kills 2 of the Sand Snakes, imprisons Ellaria and makes her watch as Tyene dies a slow death from poison in order to win Cersei's favor. When he meets Cersei, he brags about killing the Sand Snakes a lot and flirts with her pretty heavily every time he sees her telling her what a good husband he's make. Cersei however ignores all his flirting and doesn't marry him or even have sex with him.
The 4chan spoilers gave some more details saying:
"If you're talking about the sea battle of Euron vs Yara and Theon, there's no dragons, Euron just fires a bunch of fireballs and burns Yaras fleet, then gets on yaras ship and stars slaughtering people, stabs Obara in the stomach, bashes another snakes head against a railing (I think nymeria), then captures Yara, and Theon panics and jumps".
So I think this means that in episode 1:
-Yara will be taken hostage by Euron
-Theon will abandon Yara
-Theon will get the shit beaten out of him by Ironborn
-All 3 sand snakes will die and Ellaria will be imprisoned