Well that logic, though valid and good, would exclude a lot of stuff in the game.
Maybe that could be the point? I'm assuming that we're on the same page here (the same page being her level 10 power being dependant on water terrain).
In my opinion if people are going to be building Suhayr, they're going to take into account of what kind of units that the player plans to have (like corsairs for leadership) and what terrain they plan to play on. I kind of like the fact that Suhayr's level 10 power is dependant on water terrain because it's an original idea. But at the same time I can see how it would be more of a disadvantage.