AlexUM - wasn't calling you into question, just suprised..
Crash one - think it's been covered pretty comprehensively, but yes, water was causing a missfire somewhere on the system for that on one mag, hence the roughness; i.e. it wasn't working *consistently* on that mag. Eliminating it from the equation leaves smooth running, at a (very) slightly reduced power. I'm not suggesting that's normal behaviour - in this case the dousing simply upset something that was probably failing anyway.
Yes, you do get better burn with both mags, but it's perfectly possible to design an engine that burns well with one (some cars, notably alfa romeo have run twin-plug installations). A two plug system running on one will most likely be worse than a one plug system running on one, but dropping one mag out does not make that much difference - IIRC about 200rpm from the cruise setting in this case.