四分律行事鈔資持記 2.659

2.659

水界。《善見》、《母論》，約水陸兩果辨失。《僧祇》據水中明量，謂涉水者。船上即屬船界，故特簡之。

The water boundary: the Samantapāsādikā and the Vinayamātṛkā distinguish forfeiture based on the two fruits of water and land. The Mahāsāṃghika-vinaya explains the measurement based on being in the water, meaning one who wades through water. Being on a boat belongs to the boat boundary, which is why it is specifically set apart.