This is interesting. There are a lot of different accounts of the requirements around this variant and what is permitted. My own interpretation of a strict reading of the rules was that if you took the spiritual variant and boat ride you put your Compostela at risk, because you wouldn't have walked all of the last 100 km. (As we all know, it is not enough to walk 100 km; they have to be the
last 100 km. If you walk from Roncesvalles to Portomarin, ride a bike to Palas de Reis, then walk the rest of the way to Santiago, you will have walked well over 100 km but not the last 100 km, nor will you have biked the last 200 km, so no Compostela for you.)
@JohnnieWalker corrected me saying there was an official ruling and exception from the cathedral authorities that, in this case, the boat ride was permitted precisely because the Spiritual Variant is an official Camino route. When I asked him where this was documented, he pointed out that it was shown as an official Camino route on the cathedral issued credencial. With that as evidence, it is hard for them to say that it is not part of the official Camino!
In another post, I read
@Theatregal quoting him which may help sort out the confusion. She quotes him as saying:
"The discussion has helped clarify one central point regarding this Variant and the Compostela - previously the Pilgrims Office required pilgrims to walk 100 kms continuously to Santiago. They have now clarified that this variant is an exception and that pilgrims who use a boat for some of it will still qualify if they started in Vigo or Porinno and therefore overall will have walked more than 100 kms to Santiago"
So, it seems that the Spiritual Variant as a whole is official Camino and counts, but the boat ride is a weird bit. It is Camino enough to not disqualify you from a Compostela for having used motorized transit. But it doesn't count towards the walking distance.
If that is the case, much of the Spiritual will count towards your 100 km - just not the boat ride bit.
Of course, there is always the risk that the volunteer you are faced with at the desk will have a different understanding or interpretation of the rules, so make your decisions based on what is most important to you.