My mythical observation and contribution is:
Myth - It was not a boat made of stone. This was proven not feasible during the Great War (WWI) when ships were made of cast concrete, as an experiment. The buoyancy needed to navigate does not exist with so great a mass. But, things being what they are, the myth at Muxia is not going away anytime soon.
Myth - If myth #1 above is correct, then the notion of a boat self-navigating the length of the Mediterranean through the Straits of Gibraltar, up the Atlantic Coast, along the Costa de Morte (Coast of Death) into the port of Iria Flavia (Padron) is similarly improbable, albeit very romantic. But, again, be it far from me to dispel a really good story....
Legend - The two followers of St. James, Thaddeus and Athanasius escaped with his remains from Jerusalem to the, then seaport at (what is now) Haifa (IIRC). There, they obtained passage on a boat to convey the Apostle's remains to Iberia. This much was documented by Josephus, the first century historian of all things in that part of the empire... So, while I term it legend, this much
may be a documented fact... or at least a historically documented fact...
FACT - At that time in history and throughout the Roman Empire, there were boats stopping at Mediterranean ports all around the Roman Empire, delivering manufactured goods from "Rome" and bringing back raw materials or other products desired from the far flung reaches of the empire. They certainly transited the Mediterranean between the Holy Land, Italy, 'Gaul,' and Iberia (present day Andorra, Portugal and Spain), as well as North Africa. Historical writings depict a regular pattern of port calls and trading relationships within the Roman Empire.
FACT - The, then major Roman port on the Mediterranean for Iberia was the present day Tarragona (nee Tarraco). Even today, this place remains a port city.
FACT - Maps of ancient Roman roads across Spain show one major and several subsidiary Roman Roads leading from the, then major port, of Tarragona across Spain to the region of Lugo, at Asturias. Extensions of these roads connected Iria Flavia (Padron) and the region now called Galicia and that of Santiago de Compostela (nee Assegonia) by the established network of roads for carts and wagons.
Here is a comprehensive map of medieval roads, patterned upon the Roman roads. The bolder green lines are the original Roman roads. The lighter lines are the roads built later, using the Roman Road network at a 'spine.'
http://fabricadesites.fcsh.unl.pt/mercator-e/results-2/medieval-roads/
Here is a 'modern' network representation map, similar to urban metro maps:
https://sashat.me/2018/11/21/roman-roads-of-iberia/
There were even sub-roads, built during Roman times that connected Assegonia (Santiago de Compostela) with Iria Flavia to the south and Lugo, in Asturias, to the north and west.
http://dare.ht.lu.se/
The point is that the facts easily support a cart-based transit of Iberia from the port of Tarraco / Tarragona...
Fact(ish) - told to me personally by the historian of the Cathedral - Records and reasonable conjecture based on known facts, indicate that it is more likely that the two followers of St. James, Thaddeus and Athanasius, obtained passage on a boat carrying stone statues from Carrera in Italy (or similar processed stone) to the Holy Land. Athanasius wrote that the two followers obtained passage on such a 'stone boat." It is also recorded that this boat was piloted by 'a follower.' This was a boat for the transporting of stone around the Mediterranean, not a boat MADE literally of stone.
Given this, the fact pattern supports this boat eventually making a regular port call at Tarraco. From there, conjecture has it that Thaddeus and Athanasius had a 'most excellent road trip' by ox cart. Their journey eventually led them, to the first-century Roman settlement at Assegonia (akak Santiago de Compostela).
This is where the combination of facts, legend, and myth combine to form the narrative we all (well most of us) subscribe to today. The Apostles' bones were placed in a family crypt within the first-century villa, owned by one of the nine-families Santiago converted during his first go in Iberia. The rest we all know from more or less documented facts.
Assessment of known facts, lightly documented facts, and logic, permit one to 'connect the dots..." at least IMHO.
So, my submission in support of the prize, is that it was not a boat constructed of stone. It was instead a boat constructed for the transport and trading of stone around the Mediterranean.
See explanation above...
Sorry about the length. Brevity is not among my few virtues...
Hope this helps the dialog.