I suspect I could search this out in his autobiography but Twain-L may 
be quicker - Did he ever mention drinking Mississippi River water?  If 
so, when might this have occurred?  This is certainly not something 
anyone would choose to do today but in the eighteenth (pre-industrial) 
century this was considered as quite healthful.

-- 
/Unaffiliated Geographer and Twain aficionado/