Writing this might be a little tough because I don't know how much readers know about One Piece, nor do I want to risk spoiling anything big for those who have followed One Piece, I'll try to strike a balance...here goes.
One Piece is probably hands-down the most popular manga series in Japan, and probably one of the more popular series in the US. Very briefly, it takes place during the "Great Age of Piracy", where different crews from different ships are searching for a great mythical treasure left behind by Gol D. Roger, called the "One Piece" (hence the title of the series).
Naturally, since this is a Shonen manga, many of the pirates are larger-than-life and have many superhuman abilities, which makes for some great fights and unique set pieces. The series primarily focuses around the crew captained by Monkey D. Luffy and his pirates, the Straw Hat Pirates, named such because of the straw hat worn by Luffy. Unlike the typical 'bad pirates', Luffy's crew are what are known as Peace Mains, which basically means they (generally) they don't loot and plunder the innocent and are often fighting on the side of good (if you're going by D&D Alignment rules, Luffy's crew would probably average Chaotic Good). Despite that, the Navy, as well as other pirates, are often always on their tail, which makes for some really cool fights.
Moving on -- several issues back, the series went through a timeskip, and the arc that's been going on for the last volume or so has been, in my opinion, rather lackluster. I honestly have found it hard to follow the plot of this arc, which seems to revolve around two types of fish-men (these are the species which includes mermaids and mermen) -- those who want to have peace with humans, and those who don't. Predictably, those who don't want peace with humans are violent and terrorize those who want peace, and it's up to Luffy and the Straw Hat Pirates to help balance the odds.
This same sort of plotline has happened before in One Piece (and is probably a staple in Shonen manga), where the protagonist and his team come across a village that's being terrorized or is in the midst of a civil war, the hero goes in with his team, fights it out, and makes the village right again. And this has worked incredibly well for One Piece in the past, especially since the world that Eiichiro Oda has penned is just so creative and fantastical, but it falls flat during the Fish-Men arc.
I think the primary reason for this is because it took place after a timeskip, and the one thing we all really want to see is how strong the members of the Straw Hat Pirates have become. I almost hate to say it, but Oda seems to know this, and is doing nearly everything he can to keep from having to show this. In previous volumes we saw a few one-off shots of their power (Oda used a pre-timeskip villain in order to show their post-timeskip strength), but we haven't seen a straight-up "all the Straw Hat Pirates vs. all the bad guys" fight in so long! Another disappointing thing is that several fights (although it was worse in volume 63) occur off-screen and you see the Straw Hat Pirate in their victory pose over the nameless villain -- apparently they've gotten so strong their fights can't be animated!
Furthermore, it's very hard to care about these characters. Oda managed to make the persecuted people of past villages so likable and whatnot that you got a little attached to them and hated to see them tormented by the baddies. The peace-loving fish men in this volume are so generic and too numerous, for the most part, they lack distinctive personalities and those little idiosyncrasies that make characters fun to relate to.
Some of that might be forgiven if it was only a chapter or two, but for about half the book time is wasted showing fights which don't make too much sense. We already know the bad guys are bad by the stuff they've done both in current time and flashbacks, what good does it do to show us a bunch of people we don't know (nor do we know their strength) fighting and losing to the bad guys? For a disturbingly large number of pages I was just flipping through skimming as somebody beat up somebody and other somebodies were shocked and amazed at this.
The book makes a turnaround about halfway through, when we finally get to see the Straw Hat Pirates join the fray. Once this begins, those fight scenes, while decent, seem a bit stiff for Oda, definitely not up to his usual quality, but towards the end of the volume, I think he starts to get it back, which makes me actually interested in volume 65.
If you haven't read volume 63, I have to say you really aren't missing much, you could probably find a chapter-by-chapter summary guide online and just read those for the missing chapters, however if you are a fan of One Piece and you haven't picked up any books since the timeskip, I'd say start with 64, because I have a feeling we're in for some good stuff come volume 65.
This volume has chapters 627-636 (inclusive).