四分律行事鈔資持記 2.1049

2.1049

不犯中。律云直渡彼岸也。（但開過岸，非謂遠途。）失濟者，本為直渡，上下非意。又云：往彼岸不安隱（却返上下），勢力繫閉命梵等，皆開。

Among the non-offenses: The Vinaya states, 'crossing directly to the opposite bank' (it only permits crossing to the bank, not travelling a distant journey). 'Losing passage' means that one originally intended to cross directly, and going up or down was not the intention. It also states: 'going to the opposite bank is not safe' (so one turns back, moving up or down), or being constrained by force, or difficulties regarding life or brahmacarya, all are exceptions.