I keep reading posts blaming Imry with a bunch of failures on the Blackwater. I don't get it. Why does Davos saying that he would do things differently mean that those things are right?
Consider that Davos is a captain of a war galley, but he was never in a naval battle on this scale, only the occasional small scale fights back in his smuggling days.
If we look at the decisions:
1. Does'nt send scouts. Davos would have stopped, sent scouts, and wait for the information.
2. Ignores the chain. Davos would have stopped, and offered Stannis to land around to land north of KL.
3. Places the pirates as rear guard, and not at the front. Davos would have used them first, since they are good fighters.
Now, Davos' choices are all dead wrong, and would have ended with defeat all the same.
Consider the timeline: Davos needs to shield his eyes from the westering sun, meaning that it's late in the afternoon when King's landing is in sight of the fleet. Garlan leads the van into Stannis' host at dusk. That gives the entire battle that we see, from Davos' chapter and until Tyrion is attacked by Moore about 3-4 hours total. Sansa is in her room all night after leaving Cersei, and only hears that the fighting south of the river is done at dawn, a good 12 hours at least later.
Davos's decision to send scouts would mean that his ships are wasting time going to see... exactly what they were expecting to see. 50 enemy ships, only with 3 missing ships (King Robert's Hammer, Bold Wind, and Seaswift, who are escorting Myrcella to Dorne), and an assortment of smaller ships filled with soldiers. The Wildfire is hidden in ships, and scouts don't get x-ray vision because they are scouts. The Wildfire is also not a known threat, since news of the dragons have not reached Westeros yet, and Jaime never bothed to tell anyone about the caches of Wildfire that are hidden in King's Landing. So there is no way to know that the Pyromancers can make so much.
Davos' offer to go around and land troops north of King's Landing, assuming he sticks to his guns and Stannis agrees, means that the fleet would take as many troops as it can, to make the trip. Saan has cogs in his fleet as well as galleys. He only managed to cram 1,300 armed men and (assuming no great losses to Wildlings, I'd go with Massey's figure) 800 horses. Clearly, there is'nt room enough for the entire army. So the army would need to split up, and while half is going north, the other half is destroyed by the arriving Tyrells, and lack of evacuation option.
Davos' wish to see the pirates at the front is simply flat out void of logic. The pirate ships are not all galleys, and the galleys are smaller than the Westerosi war galleys. The galleys would be smaller than the enemy's, and would likely lose in a fight that is focused around ramming the other ship. Not only that, but even the ships that survive first contact would be facing more enemies, since the Westerosi war galleys are larger and can hold more soldiers. As good at fighting as the pirates may be, attacking a larger group of soldiers is simply a bad idea.
So, none of what Davos offered is usefull.
From Imry's POV:
If half the fleet gets cut off, so be it. They have enough at the front to destroy the enemy, and the galleys could ferry troops while the rest of the fleet goes the long way. 3 enemy ships are missing. If the return, Saan still has enough ships to defend the rest of the fleet. They have already wasted days because of the storm, this plan is simple and there are no legitimate reasons to stop and waste more time, or to place the smaller pirate ships at the front.
Now, I get that choosing Imry was done for political reasons, but considering the knowns at the time, it realy seems like he did everything he should have done. He was placed in a position that should not have failed, and it didn't. Stannis' fleet was defeating the enemy, and then fire ships showed up and fucked shit up. For everyone. Lannister ships burned as well, and only 12 (including the 3 that went to Dorne) are left after the battle. That's 9 Lannister ships, and 30-40 of Stannis' ships upriver of the explosion. Imry was winning until the plot-device blew up in his face. Nothing that Davos would have done would change the defeat. At most, Stannis would still have a fleet, and a slightly larger army than the one he had earlier in the book. But he would not take King's Landing, and he would not have more options after the Tyrells show up.
Should Stannis have picked a different commander? What would a veteran captain from Fair Isle would have done? Nothing more than Imry. There is no real reason to see Imry failing with 4-1 the ships. Why not give him that role, while giving the important roles to people Stannis can trust? An army 16,000 strong with all the nobility of the south, and a bastard (known as a seasoned killer) leads a battle group, while his brother the actual lord does not lead one? How does that make sense other than giving important roles to people who deserve it?
TL:DR - Imry got a job he could'nt fuck up as reward for his house, and did good. Davos would have made pointless decisions that would change nothing substantial, and would take away from Stannis empty honors to give to his followers when he needed those.