If I'm right, it was holding a Utility Umbrella. As per Bulbapedia:
The Pokémon holding Utility Umbrella is unaffected by effects of rain and harsh sunlight. All effects of sandstorm and hail work as normal. Water-type and Fire-type moves targeted at the holder will not have their power decreased or increased in rain or sun.
252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 252 HP / 252+ SpD Blissey: 262-309 (36.6 - 43.2%) -- guaranteed 3HKO
252+ SpA Choice Specs Kyogre Water Spout (150 BP) vs. 4 HP / 252+ SpD Blissey: 262-309 (40.1 - 47.3%) -- guaranteed 3HKO
Without the effects of rain, it is a 3HKO, as seen in the above calculations.
Hope I helped :)