Hard Brexit and travel for UK citizens into Schengen countries in 2019

I am a British citizen and US permanent resident. I rarely need visas. However, I am considering a Mediterranean cruise in the spring or summer of 2019.

Unlike British politicians, I like to plan ahead and be prepared. If I am going to need a Schengen visa, I want to assemble the documentation early next year, so I can apply to the appropriate country as soon as I have an itinerary and it is at most 90 days before travel. Given how things are going, I am preparing for a hard, no-agreement, Brexit.

For trade, that would mean WTO rules. For personal travel, is there a similar fall-back that would allow me visa-free visits to the Schengen countries?