The Dive Ball is a PokéBall that works best on Pokémon when you are Underwater having used the HM move Dive in Ruby, Sapphire & Emerald. In later games, the Dive Ball is more effective on Pokémon who are found in the water while Surfing or Fishing. When Underwater, Surfing or Fishing, the Capture Rate of the Dive Ball is 3.5. At all other times, it is 1.
CatchValue = ((( 3 Max HP - 2 HP ) (Catch Rate 3.5 ) / (3 Max HP) ) Status Modifier
Serebii says the exact same thing, so I doubt it is a typo. The only logical conclusion this data provides is that the multiplier does not apply to Pokémon caught while fishing or Surfing in RSE/ORAS.
TL;DR Due to the information being the same on two reputable websites, it is true that the Dive Ball multiplier does not apply to fishing/surfing in Gen III. This is true in ORAS as well, being remakes.
Also there are no Pokémon you can catch in Gen V while diving except Lugia. But, before the battle starts all the water dries up so you're not really diving anymore.