Yuan-Ti
The reinvisioning of the Yuan-Ti is BRILLIANT!
Out go halflings. Never liked the little kicktoys anyway. Every player made them Tasslehoff knock-offs.
The primary waterway traders are now Yuan-ti! Sure, noone likes the Yuan-ti. They are suspected of all manner of evils. Noone trusts them. But water travel and trade are too vital to the world and for some strange reason *cough sabotage *cough none of the other races can get a foothold on the river trades. The Rhennee from Greyhawk followed this model.
Long ago the Yuan-ti figured out that lying low and plotting from abandoned temples was nice and all but really, if you want to rule the world, what better way than to control commerce? So for generations they worked to "deter" the other races from plying the waterways. They succeeded, sinking non-yuan-ti craft, subverting crews, or just flat out buying the competition over generations until the only waterway merchants worth mentioning are primarily Yuan-ti or controlled by Yuan-ti.
Now the Yuan-Ti are the undisputed masters of all things aquatic. And their influence can spread because the world NEEDS them. "Who better to ply the dangerous and scary wilderness rivers than snakes?" say the typical human townie. "Glad I am I don't have to ride the rivers. Too dangerous for decent folk. Let them get gobbled up if they want it so bad!"
One problem with the halflings-as-river-folk thing: halflings are essentially snack-size for most river predators. And that's just the natural predators. Now throw in scrags and other aquatic nasties and the poor little yummies don't stand a chance. Aren't they caramel flavored?