So what did he do? He directed in his will that $5,000 be used to construct a water fountain in downtown Detroit, making available "water cold and pure as the coldest mountain stream". I didn't make that up, that's what his will said, at least according to Wikipedia.
So his family later commissioned noted sculptor Henry Hobson Richardson to design the fountain, which he did. In 1887 the Bagley Memorial Fountain was unveiled at Woodward and Fort Street. In the 1920s, traffic got a little thick so the fountain was moved to Campus Martius, where it stood until 2000. Then somebody stole part of it (apparently they were very thirsty!), so the city removed it and put it in storage. In 2007, the fountain returned, and it now stands in Cadillac Square, just down the street from Campus Martius Park.
By coincidence, the fountain now resides across the street from the Greenwich Time Pub, which happens to be a great place to go when you're thirsty.
And in case you're wondering, those guys in the background of the photo? They're playing petanque, and if you don't remember what petanque is, click here.