Nautical reference

A complete sea change?

A sea-change wouldn't describe this situation. It is a significant change in the nature of something;

"Full fathom five thy father lies
Of his bones are coral made
Those are pearls that were his eyes
Nothing of him that doth fade
But doth suffer a sea-change
Into something rich and strange."

Ariel in The Tempest by William Shakespeare

I can't think of a nautical reference; it is a demonstration that Little Things Make a Big Difference and shows the danger of extrapolation.
'A stitch in time saves nine' might also apply - an early correction to a course would prevent a long diversion.

Plain/plane sailing is also a reference to navigation at sea.

Henry

I've always understood it to relate to the way the sea and the weather are interlocked. Sometimes the sea can be very rough and then, suddenly, calm. This often is due to a change in atmospheric pressure and denotes a major shift in things, best shown up by the state of the sea.

See also: the meaning and origin of the proverb 'a stitch in time saves nine'.