think about it, when the solar eclipse accures, the FN wont be able to bend fire... but why not? because the radiation (if u can call it like that) of the moon blocks the one of the son. remember the Siege of the North? the water benders gained an extra power boost in their water bending during the night, and this was because of the moon. when the FN will get invaded, the moon will also be high at the sky, blocking the son. this means that the powers will be unbalanced for a while, which means that the Water benders will probably be more powerfull and any other nation's element. ofcourse they need the help of the EK, but the help of the WT is more important if u ask me!

I said this some where else, but here's my take on it:

I think that in the end, Avatar will pull a "Lord of the Rings". While Aang and the gang go in to confront the Fire Lord, all the other nations will launch this huge attack and invade the fire nation during the solar eclipse. I think it would be cool if everyone we had seen before had come back to fight in this final battle. People like Suki and the Kyoshi Warriors, (if he survived) Jet and his Freedom Fighters, maybe General Fong and his men, and Pakku with some waterbenders, and hey, even maybe the deserter, Jeong Jeong and his men, who knows? I think that would amazing if this happened. and this way, we do have some waterbenders fighting if they do indeed get stronger during a solar eclipse.