The river section, sometime also known as the Via Crucis, starts in Vilanova de Arousa and goes to Pontecesures on the Río Ulla, to be followed by the short walk from Pontecesures to Padron.
Taking the river section does qualify towards gaining the Compestella, and the operators will stamp your pilgrim passport.
There are 3 operators on this route, each departing at slightly different times, with departure times influenced by the prevalent tides.
In no order of preference, as the prices from each operator are the same, currently 20.00€ per person one way only, these are the operators
La Translatio / La Barca del Pergrino / Naviera Bahia Sub
tel : 00 34 607 911 523
naviera@bahiasub.com
https://www.labarcadelperegrino.com/
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Translatio Xacobea / Tours Rías Baixas
(they operate the fast rib boat/launch)
tel : 00 34 680 733 990
info@toursriasbaixas.com
https://www.toursriasbaixas.com
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Amare Turismo Náutico
tel : 00 34 650 410 322 + 650 410 323
info@amareturismonautico.com
https://www.amareturismonautico.com
Remember that departure / sailing times vary as they are dependent on the tides.
Hope that this might assist