So in talking to freedom, robby, and robo last night we got into a discussion on what Fishman Island is missing. I know this has come up before, but up to this point most of us wanted to wait to see where this arc was going to comment. Now, there still could be a huge twist, like I hope, that will push this arc in a different direction. However, the beginning of the arc simply didn't have the same kind of gripping power that most other arcs have had, and that is never going to change to matter what comes later. To summarize, I think it comes down to characterization, or lack thereof as a result from characters being spread too thin due to the sheer number of them (in the flashback), and possibly a sort of oversight with Hody. This arc also lacks danger, but so did Bellamy and he was fine, so I've decided that's not really the issue here. I think it comes down to the development of Shirahoshi and Hody most specifically.
First I'll use Drum as an example. Chopper was used as a vessel in his flashback to introduce and connect with Hiluluk. This is a big reason that made Hiluluk awesome. He's a great character, but you also get emotionally tied to him immediately. Now, the Fishman Island flashback had Jinbe and Shirahoshi at best for that role. Shirahoshi wasn't even present for Otohime's introduction as she was just a newborn then. Otohime actually had a pretty cool intro from her side, but she had to do so along side random people who didn't even have names. Really, Otohime stood on her own throughout the majority of the flashback, and only used Shirahoshi for her own personal inspiration. As a vessel to connect us to other characters in the flashback, Shirahoshi was nothing short of terrible, and Otohime didn't really have anyone better. I've only really started connecting with Shirahoshi as a character fairly recently, which is too late for Otohime.
Tiger was a tragic character that actually had a lot to like, and he had Jinbe as a vessel to connect to him with. His biggest problem I think is the lack of a scene to hook the readers. Rather we got a series of summary boxes that gave us an uninspired overview of his past with Jinbe and Arlong. I suppose it was done to save time, but expanding that out and showing us why they loved him could have done wonders for his character. It was by far his best chance to really connect with Jinbe and Arlong, and thus connect with the readers as well. Then the remainder of his portion of the flashback would have ridden on that. I'm sure he would have gotten the proper development had he not been forced to share the flashback with Otohime, and I guess Oda didn't want to make it as long as necessary to properly develop both.
The other major problem character in this arc is Hody. Yes, he's been sufficiently dickish for a villain, so that's not the problem. It's true he doesn't elicit much of a sense of danger, but I don't think he's doing any worse than Bellamy did, so that's fine too. His problem seems to lie simply in lack of success (and most of it fell in his lap). We haven't really seen him actually do much to make us hate him. We saw him shoot Shirley (who provoked him), and we saw him string up and pelt the royal family with water, but that about sums it up and both of those things are quite recent. The rest of his feats have occurred off panel. We know he shot Otohime, but nothing was drawn to give us memories of it. We know him and the officers defeated the princes, but it was all off panel. He had a cool scene destroying Gyro and messed up a lot of nameless pirates and used his crew as a shield, but most of us don't care about any of them. A scene in which we see him viciously maul a character we actually care about (or even some innocent civilians) would have done wonders for him as a villain, and I think that alone is his biggest issue. The hook was never actually set, so to speak, as he talks a lot but fails to back it up with his actions. It sort of started changing in the last 2-3 chapters, which might have been too little too late, but I don't really know about that yet. He's had several chapters to himself pretty much, but has failed to do much more with them than be an ass with a sick vision.
Decken, on the other hand, has been much better. Him terrorizing Shirahoshi for 10 years and the way he acts about it serves him well as a villain. This would be true even if he never tossed Noah and never managed to be much of a threat at all. I give him the thumbs up as a character.
Caribou has also been good. His one scene of him capturing innocent mermaids made me hate him as much or more than Hody despite Hody's several chapters full of opportunity. I give him the thumbs up as well.
Tl;dr I guess, but it was on my mind.