When are Dive Balls extra effective?

According to Bulbapedia:

Effect: Allows the player to catch wild Pokémon; works better on Pokémon encountered underwater.

As you can see here, in ORAS Dive Balls work the same as in third generation.

The enhanced catch rate is not specified for ORAS, but is reasonable to think that it hasn't changed.

Based on my testing in Alpha Sapphire, Dive Balls appear to have the 3.5× catch rate for all water-dwelling Pokémon, whether encountered while surfing/fishing or underwater.

First, I tested out Dive Balls on Magikarp at full HP, with no status conditions. I threw nine balls while surfing and nine while fishing. All 18 balls successfully caught the Magikarp (~100% modified catch rate), which is consistent with Dive Balls having a 3.5× catch rate. If Dive Balls were just 1x effective, it should be a ~33% modified catch rate.

I also threw nine Dive Balls at underwater Chinchou (also at full HP with no status conditions). Chinchou's catch rate with a 1× ball is theoretically 24.8%, and with a 3.5× ball 86.8%. Eight out of nine were successful (~89%), which implies that Dive Balls are 3.5× effective underwater as well.