Oliver Cowdery initially wrote “here is the water of Mormon”, but somewhat later he corrected the singular water to its plural, waters (the level of ink flow is uneven). This correction is undoubtedly correct. All other references to “the waters of Mormon” (seven more of them) have the plural waters: Mosiah 18:16, Mosiah 18:30 (three times), Mosiah 25:18, Mosiah 26:15, and Alma 5:3. Moreover, the plural verb form were in the next clause of Mosiah 18:8 (“for thus were they called”) supports the plural waters. Perhaps Oliver’s initial use of water was the result of his having just written the singular is (that is, “here is the water of Mormon”). As discussed under 1 Nephi 4:4, instances of subject-verb disagreement are common in the original text; for a complete discussion and listing of examples, see subject-verb agreement in volume 3.
Summary: Retain the plural waters in the phrase “the waters of Mormon” in Mosiah 18:8 (and elsewhere in the text).