Clear blue water the ideological gap between the British Conservative and Labour parties. The phrase, from a blend of clear water, the distance between two boats, and blue water, the open sea, with a play on blue as the traditional colour of the British Conservative party, is recorded from 1994.
clear the decks prepare for a particular event or goal by dealing with anything beforehand that might hinder progress. In the literal sense, obstacles or superfluous items were removed from the decks of a ship before a battle at sea.
See also out of a clear blue sky.