A fleet is essential. Dany still needs to move troops to the mainland of Westeros, for one thing. But there's also the issue of where she'll land. Does she link up with the Dornish and/or Tyrells? Does she hit the Stormlands where Stannis was based, and push up from the south? Does she work her way inland towards Kings Landing starting at Crab Key? Does she sail all the way to the Westerlands and attack Casterly Rock to draw off Lannister forces? Or does she just go straight for the jugular and hit Kings Landing? Or does she split her forces? There's also the issue that, if the Iron Fleet hits Dragonstone, it could effectively bottle her up on an island that is, essentially, incapable of feeding the army she has housed there. That would lead to starvation and desertion, and would end her invasion before it happens. Essentially, if Euron lays a naval siege on Dragonstone, she's screwed. Of course, Dany's ace in the hole is what the Targaeryans have always had: dragons. Air support. Nobody can touch them currently, and they can lay waste to a fleet in minutes. Euron needs to find some way to neutralize or kill her Dragons if he's to stand a chance.