it seem to me that causticsoul (and watersoul, for that matter) is missing, and should have, a swim speed to go with that "you can breathe underwater" ability. otherwise, making a swim check makes no sense. why would you? to keep from drowning? the ultimate risk of failing swim checks is suffocation by drowning, which is moot in the case of watersoul and causticsoul genasi.
if i recall correctly, there are no other 4e d&d creatures that breathe underwater and do not have a swim speed. to be clear, i am not talking about creatures, like undead, who do not need to breathe, but creatures that specifically breathe underwater.
regarding the movement rules under the swim skill, they are appropriate for terrestrial creatures to reflect or represent their diminished movement abilities in water as opposed to on land. these swim movement rules should not be applicable to amphibious creatures. it seems to me that any race with the ability to breathe underwater probably spends a fair amount of time underwater and, consequently, would have a swim speed. kuo-toa and bullywugs both have swim speeds to reflect their amphibious nature; so, too, should watersoul and causticsoul genasi. if wotc believes that a swim speed as a racial trait would be imbalanced, then, at the very least, a heroic tier feat should be created to give them that option. it just makes sense from a fluff perspective.
just like the way that shifty is a racial feat for player character kobolds (uh, wait, that hasn't happened yet. oops.) ;-p
otherwise, great article.