Standard English allows both sank and sunk for the simple past tense of the verb sink. In the Book of Mormon, we have only three examples of the simple past-tense form for sink; all three read sunk in the original text, twice here in Alma 19:13 and once in the small plates of Nephi:
The 1953 RLDS edition replaced the second of the two sunk ’s in Alma 19:13 with sank, probably accidentally since the first was not changed. Unlike the Book of Mormon text (which only has sunk), the King James Bible itself shows more variation for the simple past-tense form for sink, with two cases of sank and four of sunk. For additional discussion of this kind of variation in the text, see under past tense in volume 3.
The standard past-participial form for the verb sink is sunk. As we might suspect, the Book of Mormon text consistently has sunk for the past participle (eight times). There are no examples of sank, the alternative past-tense form, acting as the past participle. For discussion of this possible usage, see under past participle in volume 3.
Summary: Retain the three instances of the simple past-tense form sunk in Enos 1:3 and Alma 19:13; there are no examples in the original text of sank, as either the simple past-tense form or as the past participle for the verb sink.