The whole "merman are the fastest while underwater" thing has to be taken into context. That logic would work only when the 2 people we are comparing are of the same level. If we compare a normal mermaid against any other normal creature of the sea (sea kings and sea beasts for example) the mermaid would naturally be the faster but once you get into beings who exceed the norm things change a decent bit. Hodi is a prime example, he gained a lot of strength and thus was able to surpass a mermaid. In this regard, anyone with enough strength should also be able to swim faster than a mermaid regardless of the norm. Luffy had his limitations even with the bubble, the sea drains his strength. That would not be the case at all with either zoro or sanji. Sanji has a special underwater movement technique and zoro already showed he could blitz hodi. I would think that either of them would be able to go out of the bubble and defeat hodi before they run out of oxygen. The issue here is that at least zoro was already shown to be strong enough to make fodder out of hodi and sanji is never that far behind. Heck, for another comparison, wasn't hyozu stronger than hodi by quite a bit to begin with? Post transformation hodi was not able to block a punch from luffy while this guy was able to do it before his transformation and even after the transformation he was not match whatsoever for zoro.
