Great shot!
You illegally escaped from a penalty shot IMHO ;) Be careful to have a rules official confirming the situation, or not to be caught after signing the scorecard! LOL
To me it looks like the ball is rolling down under the bridge from the greenside end and rests there, either on the slope or in the burn. Either way, water hazard rules apply (rule 26-1), drop at the far side from the green with one penalty stroke.
The immovable obstruction may be the surface of the bridge (ball rolling back on it?) or another artificial item (pillar?) in the water hazard. In this case (water hazard), there is no relief available (rule 24-2 b). Play it as it lies or apply relief from the water hazard as stated above.
Thus, unless special RSG conditions applying, this free(!) drop would make up for rules breach #4 in the game. (Note to myself: Start a list. Note 2: Play Royal St. Georges in natura)