Because you are playing Emerald, not ORAS, I suggest Mudkip. Why? Swampert gets the same HMs, has a much better Special stat, and STAB Mud Shot/Earthquake is beyond useful againts the likes of Steven. Plus, it only has a Grass immunity and there are no notable Grass trainers in Emerald. Azumarill doesn't have much of a niche over Swampert except being a Fairy-type and having Superpower, neither of which it had in Emerald.