Even if you were really dehydrated and drank a bit of lightly salted water,I think the dehydration is the issue,the added salt on your low sodium diet is not an issue,IMO. Doubt a salty day is going to kill you,dehydration surely could potentially take you off the life force.

As for what a triathlon athlete needs,doesnt really apply to us average schmucks,and Im sure they have experts assessing their needs,or are experts themselves for their highly unusual needs.