Here initially in 𝓞, Oliver Cowdery accidentally skipped ahead and started to write the word shore. He erased this wholly misplaced word and overwrote the end part with the preposition on. When he copied the text from 𝓞 into 𝓟, Oliver initially wrote in as the preposition. He erased the i vowel and overwrote it with an o, thus changing in to on. Despite all these efforts to get the correct reading of on in the manuscripts, the 1830 compositor ended up setting in.
Elsewhere in the text, there are examples of both “on the borders by the seashore” and “in the borders by the seashore” (as well as two examples with the preposition to and one with upon):
Although half the examples have the preposition in (six times), on is also possible (occurring two more times in the text). Thus the critical text will restore the original preposition on in Alma 56:31.
Summary: In accord with the reading of the two manuscripts, restore the preposition on in Alma 56:31 (“on the borders by the seashore”).