Hi Andrew!
You can contact companies regarding bookings the only thing i would say about booking is if you can wait till a couple of days or the day before if your going to so your Camino is not fixed by that date.
The trip takes about an hour and 15 mins and lands at Pontecessures about 3 km from Padron.
Also the one i used Bahia Sub gave me the tide time on the day i wanted to sail as the boats are reliant on the tides.
So dependent on this you might have to wait till later in the day.
My thinking on this is if you are pushed for time this might mean adding a day (early morning sailing would mean you could make Santiago that day at a push)
There is more than one company that does the trip i used this one
La Translatio / La Barca del Pergrino / Naviera Bahia Sub
tel : 00 34 607 911 523
naviera@bahiasub.com
https://www.labarcadelperegrino.com/
Contacted them by email spoke to Olga however i got a ticket from their office on the day before sailing (get it the day before) i just walked into their office which is just to the right of the long footbridge into Villa Nova de Arousa; the boat landing is about 100 mtres to the left of the same footbridge and the dock building has a huge yellow arrow across it you cant miss it.
Oct 8th when i went the boat was busy it carries 150 Pilgrims ,but the other companies are there to use.
Translatio Xacobea / Tours Rías Baixas
tel : 00 34 680 733 990
info@toursriasbaixas.com
https://www.toursriasbaixas.com
This uses a ridged speedboat and the one i wanted but newbie snafu i got it wrong!!!
Amare Turismo Náutico
tel : 00 34 650 410 322 + 650 410 323
info@amareturismonautico.com
https://www.amareturismonautico.com
This company has good reviews to!
This is the bridge to Villa Nova de Arousa (
not Viana do Castelo) the office is after the gap in the buildings to the right of the bridge.
Hope this helps
Woody