The place, where the cache is hidden, is not sewers. It is a brook which flows in the tunnel under the city. The place is not closed for public (it does not mean, that it is safe place, none underground is) and by the way geocachers are not the only persons, who is visiting it. The others are e.g. urbex explorers. If you want to see, how it looks like there, see this video on news server Idnes. You can make your own opinion, if it is safe or not.
Weather forecast had predicted local heavy rain. Not exactly in Prague, because exact location is not easy to predict, but somewhere not so far. This incident is a big tragedy, but behavior of group of geocachers was risky and unfortunately they had bad luck. The second missing geocacher has not yet been found. The chance, that he survived is almost zero.