Here the 1874 RLDS edition replaced the nominal gerundive form “the watering of their flocks” with the verbal form “watering their flocks”. Either reading is theoretically possible. The critical text will maintain the earliest reading here. For further discussion of the three types of gerundive constructions in the original Book of Mormon text, see under 1 Nephi 17:32; also see the general discussion under gerundives in volume 3.
Summary: Maintain the nominal form of the gerund in Alma 18:9 (“previous to the time of the watering of their flocks”) rather than the verbal form (“previous to the time of watering their flocks”).