In The Ringworld Throne, one of the other primitive hominids points out that if the Grass Giant king finds out something's been going on behind his back, he'll "go off like a volcano". How, exactly, does a primitive Ringworld native know there are such things? Why would their language even have a word for "volcano"?
It may have been creative license on the part of the translation device, actually referring to some sort of explosive Ringworld phenomenon. Perhaps a word for when a meteor impact forces the ground up?
Early in his explorations, Louis concludes that, because the various shallow seas are full of fresh water, he might be the only being on the Ringworld that requires salt in his diet. Right, like a man who's supposedly pursued every career imaginable has never picked up enough basic biochemistry to know that sodium chloride is vital to every organism that has a nervous system...
He's also old enough to have forgotten half of what he's learned over his long life, even with Boosterspice keeping him forever young.
The supposition that he might be the only being on the Ringworld that requires salt in his diet because there are no salten seas would only make sense if all life on the Ringworld had EVOLVED on Ringworld. By that I mean ORIGINALLY evolved from a primortal sea. While there's no doubt life does evolve on Ringworld, all lifeforms on the Ringworld originally developed on a "normal" planet. Remember, Ringworld is a construct. Of course, the evolution of life on the "normal" planets in the local system may have occurred in freshwater if the planets had only freshwater seas but that is unlikely - both because the normal mechanics of fluid/water vapor transfer on a planetary scale would preclude large bodies of fresh water but also because life is unlikely to have evolved in fresh water - the salt creates electrolyte balances that allow for the transfer of energy at the moleculer level necessary for life to exist. That is, of course, unless you are an Outsider!