Coming up for air in the canal

For days now the police have had police divers taking a very smelly swim around the grunge in the mirky depths of the canal.
They are given a bit of a wide berth by fellow officers when they emerge for air.
I don't wish to speculate but there was very recently a murder in the nearby street and they seem to be definitely intent on searching thoroughly every inch of the waters there, but no information can be given out as to the reasons for the search.