Clear blue water
Originally a term from competitive rowing referring to an obvious gap between the leading boat and those following. In more recent years it has been used allusively to mean the discernable distance between the ideologies of two political parties.
The rowing reference is first cited in the 23rd July 1870, edition of The Boston Advertiser in a piece titled 'Yale & Harvard Boat Racing':
"Lyman promptly set forty-four strokes to the minute, which brought them to the stake with an oar's length of clear water between their boat's stern and the Yale's bow."
In the contemporary allusive sense it was first used either by or about the British Conservative Party and their opponents in the early 1990s.