Pandemic Legacy 2 - definition of "adjacent"
In the rules, page 6, it indicates (emphasis mine):


  SUPPLY LINES
  Although there are no supply lines at the beginning
  of the game, cities will be connected to each other
  across land by supply lines. Cities directly connected by
  a supply line are adjacent.
  
  SEA LANES &amp; SEA ROUTES
  Locations are connected to each other in the ocean by
  sea lanes. Locations directly connected by a sea lane
  are adjacent. A series of connected sea lanes is called
  a sea route.


So cities connected by a (single) sea lane are adjacent, and the plague would spread to them as well.