BW2 might be an exception to rare fire types, because Darumaka comes pretty early and also has better stats and level up moves than Emboar. Oshawott, on the other hand, is probably not outclassed by another water.
Also this isn't relevant to BW2, but water starters were the best until Generation 4. This is mainly due to getting surf pretty early while fire starters are still stuck with ember, and the fact that water starters learn a bunch more HMs.