Yeah, I used to watch filler before finally understanding what it was - empty plotlines that will never connect to the main one. Naruto was terrible about this, because the show tried to make it seem like the filler would someday matter or payoff. After a filler episode or filler arc, characters would be like "visit us someday!!", and "I hope to see you Naruto when you become Hokage!!" and Naruto would be all "Yeah! That'll be great! Believe it!"

Never heard from or seen again.

It's not a terrible season - they just forgot to give the Leviathans enough to do.