The Showdown! command "/ds water type,surf,!dive,generation<6" doesn't find Lotad, even though Bulbapedia says it's water type, can learn surf, can't learn dive, and was introduced before Generation 6. So which site is wrong?
From what I can find, Lotad can never learn Dive. So Showdown! is wrong.
Source: Pokémon Database
According to the national Pokedex book lotad does not learn dive, but it is able to be taught Surf. So therefore bulbapedia is entirely correct on this one.
Sometimes showdown gets things wrong. So I would stick with either the DB or Bulbapedia.