Well there are probably one or two arcs after the war ends....It could be that it is the war's end, then some peace time in which Naruto learns his second element. Finally Naruto goes to fight Sasuke.
The problem is that there are relatively few people who would have a water affinity! He probably wouldn't go deal with anyone in the Hidden Mist Village which leaves Konoha shinobi...Kotetsu and Izumo are the only Water users that I'm aware of....there is Kakashi, but he has used only two water techniques...so it is possible he is poor in the art. Tobirama is dead so he can't really take up lessons!
The only other person in Konha who has had any association with water was Hinata...but that was during her training during the Part I filler arc called "The Search for the Bikouchu"...it would also be good to note she didn't use water techniques but controlled her chakra against water to strengthen it

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