You don't understand Soak's mechanics. Soak does not add Water to the opponent's type, it changes it to Water. So Geodude would be a pure Water type, and Frenzy Plant would deal more damage if you hadn't used Soak.
That aside though, this depends on Geodude's Special Defence and HP stats. We'll assume that Geodude has 0 Special Defence and HP IVs and no EVs in any of its stats, to maximise damage dealt. This would mean Sceptile deals a maximum of 64,542 damage without the use of Soak:
>252+ SpA Mega Sceptile Frenzy Plant vs. 0 HP / 0 SpD Geodude: 54880-64572 (498909 - 587018.1%) -- guaranteed OHKO
With Soak, however, Geodude takes 32,286 damage at most.
>252+ SpA Mega Sceptile Frenzy Plant vs. 0 HP / 0 SpD Geodude: 27440-32286 (249454.5 - 293509%) -- guaranteed OHKO