In the anime, water did stop flowing, Just rewatched it.
My point is that God Slaying magic is far superior to Dragon Slaying magic. The nature of the magic is completely different, with God's magic being superior. This suggests that Gods being superior to Dragons is the norm, not the exception.
We only know of one God in it's true power, Ankhseram. He can solo the verse in an instant if he wishes to. Your judgement is based on other two.
- Dimaria's was a takeover, and the power of the takeover depends on the MP of the user, see how Satan Soul's power changed over the series as Mira herself improved.
- Second one was a summoning, whose power depends on the summoner, and Arlock was weak. In fact, if we factor in his (and his guild's) weak MP and see how strong Ikatsunagi was (no sold base Natsu's attack) inspite of the weak summoners, it's logical to assume that the real Ikatsunagi is an absolute monster of a powerhouse.