Got it.
In tecnical terms: When a unit appears or is unlocked in a 'blocked' region, the game engine tries to move it to an adjacent unblocked region. In this case, that's water, with no transports, so the units are destroyed.
AFAIK, This logic is in there for units in water transit that get caught in a 'freeze'.
In game terms: Those units shouldn't even be in the scenario
Will be fixed in upcoming patch
You can continue your game, although I suspect you just took a VP hit [or the AI gained some VP

]
Thanks for the report and your patience.
