There were two issues at work here:
1) The combination of default MaximumLaunchAngle and default CruiseAltitude made torpedoes fly a ballistic curve, which combined with BoundToTerrain type made them explode 'in the air' when hitting non-water tiles. This would result in AIr returned as target type, which is invalid for torpedoes.
2) The explosion warheads were missing the Ground target type, which is actually the (only) target type that the bridge tiles you hit with torpedoes have.
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