I think that the image is depicting the Santa Maria, Columbus' main ship on his first voyage to the New World. Alongside it is one of the smaller support ships that accompanied the Santa Maria, either the Niña or the Pinta. This identification is also supported by the presence of the misty jungle-island, which we can safely assume is probably the island of Hispaniola, Columbus' first stopping point in the Americas.

Anyways, Columbus' ships are often depicted with sails emblazoned with St. George's Cross, most likely because Columbus hailed (despite the fact that he served the Spanish Crown) from the Republic of Genoa, whose heraldry featured this same cross. I am uncertain as to whether this is historically accurate, but this feature appears in almost every modern depiction of the ship that I have ever seen.

However, the cross on this person's design seems to be kind of inaccurate in that it probably shouldn't have the triple petal flourishes. The Genoese cross is generally simple, with its arms ending in either a straight edge or a bar-edge. As far as I'm aware, the only crosses that predate Columbus that feature such flowery flourishes are the Cross of Byzantine Emperor Michael I Rhangabe and St. James' Cross, and of those, only St. James' cross is really valid (being a symbol connected with the Knights of Santiago, an important knightly order involved in the Reconquista). In any case, I suppose it's fair to assume the artist was unaware of these historical minutiae and probably just took artistic license with the design.