Some of the best sailors in the world ??? BAHAHAHAHHA NO me thinks not !! The Greyjoys are kind of like Vikings with boats that can't go to far from land. Oh yes some of them(Euron) have gone far, ON LUCK !! The reason the Lannisters weren't attacked is their cities are to well protected. Come on, the longships are better suited to raids, go in, plunder, and get the hell out ! Sure they can cause heaps of trouble and distruction, but are they capable of holding anything for long ? They have their use, don't get me wrong, but they certainly aren't a superpower. Sure the pirates in the bathtub boats could take over a yaught, but only if the captain was a fat , aging, narcoleptic, drunk. Their strength is in numbers, skill, and surprise. Not the type to hold a large, well defended city.
Now, Boaz..... Yes Tywin was quite likely having some sort of discussions with Balon, but murdering him..... The Greyjoys are raiders, how would you know where one would turn up next? To murder Balon and hope that one of the siblings would feel honor bound to join your cause is just...funny. Sure Tywin would have handed Balon a "kingly title", but we all know Balon had his eye on a much larger prize. He pays the iron price, he would never take a handout from Tywin. If he was murdered by Lannisters, I would think it would have been just to remove him as a potential threat. Euron showing up a day later ? Well, Eurons own words "coinsidence".
Was it the Iron Islanders , who were the ones that removed Balon? Euron ? Likely. Victorion ? Unlikely . Asha? Doubt it. Some sort of red priestess devil baby ? Thats where my money would lie !
No doubt the longships will have a huge part to play, but I think this has more to do with horns and dragons , not Lannisters.