Aren't the Ironborn like master pirates and the best sailors in Westeros?
Not that hard to believe they staged an naval ambush when they knew the Unsullied were coming.
Look at the map and explain how a single fleet ambushes one half of a fleet, stops at Kings Landing, which based on the location of Sunspear, means they would have to go backwards to some degree, and then sneak up on the other half of the fleet that was going all away the entire continent and wait for ambushing them the day they attack Casterly Rock (although you can make a fair argument they could have destroyed them in the ocean and not even needed them to wait to hit land)
The geography and locations of the events are impossible to match up. And if you want to talk an actual naval ambush, it's fucking impossible to hide two of the largest naval fleets in the country when they are sailing for months on end. The Unsullied, at worst, would have known they were being trailed by a faster force for weeks on end.
So is Kings landing about to be beseiged by the Dornish and tyrell armies? This season really needed to be longer, I'm assuming the Tyrell army had already started marching to Kings landing? Is the Dornish army now stuck without ships? They don't have their own? I know that their leaders have been killed or captured, but the actual nations still exist in some form. The show really needs more time to show what is going on, it's getting very confusing.
What's left of the Tyrell army is either defected to the Lanisters or is dead. Dornish are out of the game with no navy to move them in place.
Again, this is all a very half assed attempt to nerf Danny and force her to use the foreign horde she assembled and her dragons.