No, you are not wrong at all, BUT. The "problem" with a water habitat is its low total capacity compared to Earth, Cold, or Metal. So your water habitat needs emptying of gold every 18 or so minutes with Pans. That's fine when you are playing the game, but rubbish when you are sleeping, eating, working, or at school! You are not earning any more gold if a habitat is maxed out / full of gold. With the metal mine you only need to come back to the game every 8 hours or so to empty it and be always earning gold. Thats the balance you have to strike if trying to maximise gold production!