No boat to Arcade from O Porino: that is inland.
For the Variante, look for flyers in the albergues, they were in just about every albergue when I walked in 2016. It splits from the central some 5 km after Pontevedra, after you walk along the train tracks along a very straight path. You cannot miss it.
The boat is from Villagracia de Arousa to a couple of km before Padron. It all gets organised by the albergue, and leaves at times depending on the tide.
https://www.google.ca/search?client...wUIGCgA&biw=1024&bih=672#imgrc=0Xzx3Sr3rwmkXM: