Hi,nice to meet you. Now to answer the question:
Judging by BW episodes in order to accomplish her dream, Iris would have to raise and evolve dragons becoming one day good enough to understand hearts and feelings of dragon types.
With Misty she has more complex dream wanting to become something much higher(best water trainer in world),which is basically becoming one of E4 since they are best mono trainers.That and with Whirl Cup it was showed how there exist tasks you need to enter expanding abit on whole term.Episodes like "Around the Whirlpool", "Dueling Heroes" or "Perfect duel" further touch this, though it wasn't cleared enough.
It was established in anime how there exist different meanings for one type masters.Like "Water pokemon master" being explained to be strongest water pokemon trainer in world with prestigious title being given to those who proven themselves worldwide among other water trainers,while with Iris goal it was showed how "dragon master" means very specific title which is given to those from dragon village who have proven themselves to be worthy of it by having immense knowledge and expertise about dragons.
As such with Iris career as showed with Drayden you can be both gym leader and dragon master. But with Misty career which aims to be strongest water trainer in world with water based events shoeing how there exist specific tasks to conquer like coordinators do with contests gym position won't make her fulfill her quest. Especially when she was gym leader before meeting Ash returning back only because of useless sisters.
Someone like Wallace could probably be closest to water master we ever saw in pokemon.
p.s. You see dragon and water master are similar in name sense. But in meaning and what is needed to accomplish it, they are quite different from one to another. Similar how word "light" can have different meanings either being opposite to heavy, or referring to term "bright". Hope i helped and cleared things up for you.
Thanks, though I do like other previous series better, this still was a great series. The battles were good, and there were plenty of characters to go around, along with there being some unexpected surprises. I'll admit, there are some things I disagree with in the series (such as Unfezant not getting rid of Gust of Air Cutter yet), but they don't have anything on the series as a whole. Thanks again!