There is one. Click on rankings, then click on the tab at the top labelled "Alliances (Ocean)". It will automatically select the ocean for your current city, but you can input a different number in the box next to the world "ocean" at the bottom of the window and click the curved arrow to give you the results for that other ocean that you have put in.
This is the current ranking for O56 in Selymbria:
Just to note, this will tell you the biggest alliances, but not necessarily the strongest alliances, as that also depends on player quality, diplomacy, etc.