There are discriptions within the pages of the Book of Mormon which do suggest that the sea was upon the north, south, east and west, literally surrounding the land excepting (Helaman 3:8, Alma 22:33). Could there have been a 'first un-noticed' narrow neck of land of land between Jacob's and Nephi's isle? With such a possiblity, we do not have to physically identify an isolated island, but a 'cause-way' connected land to another and possibly greater land. In another presentation it will be further considered that the lands of South America were not 'one land' at the time of Nephi and Jacob, and that the actual sea west could have been an 'Amazon Sea'. From south Peru/northern Bolivia about Lake Titicaca to northern Colombia with the Pacific to the west and an Amazon Sea to the east, both Nephi and particularly a younger Jacob could have traveled that land to the extend that the conclusion was more than scriptural, that it could well have literally geographically ascertainable.
The Jaredite description of the narrow neck of land, were the land north and the land south were 'attached', was actually 'by the place where the sea divides the land' (Ether 10:20). There are instances and occasion were one body of land is 'attached' to another body of land, by a strip of land which either seasonally or according the the tides or even according to storms, may become separated when upon those occasions the two lands actually become 'divided by the over-running sea'.
At the time of the Jaredite description, the pronounced connection does not have to be as great as it was in the time of Nephi and Jacob. By their time the land connection could have been more perminated and merely missed by Nephi and Jacob as the Lord first had them journey south to the land of first inheritance and have been more of a presumption than necessarily totally factually varified though mostly factually supported by the facts of seeing all the other waters surrounding.
Of course there is always the fall-back generalization, that the people of the 'Asian' world of Jerusalem to Babylonia and beyond, felt and understood that other lands, particularly those traveled to by sea, were considered to be 'isles of the sea'. And that would even included the penicila lands such as Rome/Italy, Greece, Ibaria/Spain and even the northern shores of Africa; not to speak of the rest of Africa which they could actually sail around in their ships of Tarshish; and of India and so forth.