does water rid sodium?

Yes and yes.  This is why salt is not a bad guy.  Constantly put up against sugar and regular sugar actually kills you.  No question which is worse. 

Water helps flush out retained fluid, that can build up due to excess sodium. The RDA of sodium is just around 1 tsp/day. You can't eat all the salt you want and expect it'll be flushed from your body if you drink a lot of water. Unless you consume huge amounts of water (more than most people would ever typically consume) OR you lose a lot of fluid via urine and perspiration, should you need to consume extra electrolytes (including sodium) to keep from having issues.

It does, to an extent. But the kidneys are specifically designed to recycle and reduce the waste of sodium (among other electrolytes). And they're pretty good at it. So good, in fact, that excessive sodium is a problem in our diets. 

Inevitably, the more water you drink, the more sodium you will lose but not to the point that it flushes enough out to make a significant reduction in complications of hypernatraemia. It's easier to just not consume as much salt. Simple.

Hyponatraemia isn't really an issue unless you eat barely anything or are a strict vegan. Sodium, while fairly rare for land dwelling organisms, is fairly abundant in modern food. As it's added to just about everything.