Elven Sails then Yeah, that only allows them to travel two water trails, and there's no water trails without encounters. If they also add a Triton Figurehead, it's possible to choose a trail with just one die, and then modify that so they do not have to roll a die - technically, they have to roll, but they have to do it with zero dice.
Like that, it is indeed possible and legal to travel two trails without rolling.
For reference, the Elven Sails text also mentions: "They only make 1 encounter check, choosing which trail they are traveling on to check for encounters."