@RobbyBevard:
Why would Oda-san bother showing he can walk on the sky?
Same reasons. Show the results of his training, give them something to do on the way down to FI, and to hold them over for however many hundreds of chapters it'll be before Jinbe joins officially after the Big Mom arc.
Sanji's Blue Walk was still shown to be insufficient is some cases where limited time was a factor, like when they only had a few seconds of air to fight the kraken, or when they entered FI and got caught up in the current, or even the final battle of the arc where he didn't swim to help at all.
So if those are non-things, then why is two guys moving around well in water a thing?
I hope I don't stick my foot in my mouth with this one but here it goes. The reason why swimming is a "thing" as opposed to swordsmanship and super strength is because neither of those things have never been opposed upon.
What I mean by that is swordsmanship is a core part of Zoro's character and move set, Brook even though he's a swordsmen, his pride is in his skill to play music. After the time-skip all of his abilities were extensions of his devil fruit powers. As far as we've seen he's swordsmanship and musically ability haven't changed.
Now when we compare Jinbe and Sanji, Sanji's move-set unveiling opened with the walking abilities. I believe showing abilities such as these so early would suggest that they will be used often and increase in strength. You could also make the argument that Sanji was foreshadowed to either have this ability or at least contend with a Fishman underwater by his fight in Arlong park.
When it comes to Jinbe his pride lies in being a fishman, and one of the ability Fishmen excel in is swimming. So now your left with two people who are very good swimming. You and others may not see the logic in this, however to me, this causes an ability conflict, but hey we'll see what happens.
Also, is everyone aware that we have not seen a character who is better at Fishman karate than Jinbe? This kind of makes him the Mihawk of fishman karate,no? I mean surely if someone was stronger than him at the craft we would have heard of him or her already, just like how Jinbe was foreshadowed in Arlong Park.
(Power rambling)
Personally I don't see how someone who is the "best" can get any better. Its like there's no "surprise" in the character anymore. For example when Mihawk cut that ice berg, the reaction would be "oh damn he cut an iceberg" but knowing the feats that occur in this series and Mihawk's title the reaction would "Well no shit he cut an ice berg he's Mihawk!"