Please, explain how.

Because the rangers destroyed the fleet, destroyed the emperor, released all of the powers, and then went on a new adventure. The ending was satisfying and complete.

Would you really wanted to see the rangers hijack each ship to destroy each ship? That would get boring.

That would make it DBZ. And none of us want that. Lather, Rinse, Repeat.

Wrapping things up isn't ending perfectly. That just means they had a checklist of things to finish and they got to them.

They destroyed the fleet with so little effort it was embarrassing.

We saw so little of Ackdos that we never got a sense of how strong he was. Yes, he could hold his own against the Rangers during the one fight we ever saw him in, but because we never saw him handle a conflict earlier (And his own son wasn't much of a challenge), the final fight carried no weight. Maybe if Ackdos had taken action earlier and trounced the rangers in a fight like Basco did (Or had they made Walz a threat, to make his father's entrance all the more threatening), the victory would feel like something worth getting pumped about.

They released the powers of the Ranger Keys. This leads to my biggest problem with the series. They began with all but like 10 of them. If we saw their adventure and search for the actual keys, their release would hold some weight. But because they didn't, they giving up the Ranger Keys didn't have any emotion at all.

The ending was complete, but only because they didn't do anything throughout the series.

Did I want to see them go through every ship and destroy them one by one? No. But that doesn't mean the ending wasn't pitiful.

How would you then have had them destroy the fleet?

That is because Ackdos is an emperor, not a fighter. It carried no weight because the rangers were not going to let him even have the chance to use his powers. I mean seriously. How many times do you want to see the rangers lose against the main villian and then have them get that final energy to defeat him? We know they are going to beat him, so why beat around the bush?

Were you expecting them to keep the keys? They gathered the keys, got what they needed, and then gave them back. What more do you want? It's like you were setting yourself up to be disappointed no matter how it ended. Would you rather it had been like Goseiger where there was no final fight at all, they just had to stop drills from reaching the core?

If you think they did nothing throughout the series, you need to either rewatch the series, or watch DekaRanger and Shinkenger. Nothing happened at all during those series. In fact, the villians had even less development in Shinkenger than Gokaiger. Barizorg had more development than the entire Shinkenger villian cast.

Don't get me wrong. I loved Gokaiger. It was a great series and I would gladly rewatch it.
But it's ending was on par with Double's in terms of terribleness and it doesn't quite deserve all the praise it gets.

An interesting point. I am watching Boukenger and am up to like episode 18 of Gokaiger, and I have to say that Boukenger definitely has the "sense of adventure" you refer to, while Gokaiger does not. A lot of the time (so far) on Gokaiger they seem to be waiting for something to happen, while on Boukenger they are actively pursuing new Precious and other objects.

I like Gokaiger, but I am not in love with it. The meka aspect is very weak, and the story seems to lack forward momentum. But I am typically entertained by it, so I can't criticize it too much.

Well, keep in mind that overall Gokaiger is more of an anniversary series than a pirate series. While Bourkenger was also an anniversary series, it did not incorporate tribute episodes from past sentai series like Gokaiger did. The occurrences of those tribute episodes obviously hindered its ability to tell a cohesive story because they have to have a new encounter with a sentai alumni almost every other episode, and while they tried to tie them with their search for the Greatest Treasure, ultimately it feels great at times and also jarring in others. I think Gokaiger started off great, but got slowed and even weaked in the middle, before Ohranger's tribute put it back on track for a great finish to end the series.

If you're calling Basco's end the end of the series, I'll agree.
I don't see how the canon ending is anywhere near "Great" on the non-existent scale for endings.

I know the ending is decidedly mixed for some fans out there, and the Emperor was more like a glorified punching bag than a real threat in the finale. But I like the fact that Gokaiger rejected the Greatest Treasure to resolve the issue their way, and the way they went off on the Emperor with those Gokai Changes was simply spectacular. For a series that is ultimately about paying tribute to past sentai series, I think it ended appropriately and with alot of fireworks. I can still watch ep. 51 and be entertained by it even after watching it several times, because as far as Super Sentai goes, it couldn't get better than that visually.