A few times I have done the coastal route from Porto, walking 4 ½ days on beaches before swinging right at Caminha along the Minho valley and joining the main camino at Valenca do Minho.
Walking on the beach sounds lovely but can be hard going. I mostly walked in the harder sand in the swash zone where the waves break but then I had to sometimes run up to avoid getting wet. It was easier in the wet sand than the soft sand or on the dunes above the beach, which can be hard going.
I work old sports shoes with no socks, these proved for me to be the best option, though I did sometimes walk barefooted and in “flip-flops”. In those 4 ½ days I only saw 1 other pilgrim, an adventurous American lady who had a pack bigger than mine and looked quite prepared.
There were locals around in the villages, as usual with good advice of places for lunch or snacks. I once walked when it was high tide. This wasn’t a great idea, when squalls (showers) lashed in from the Atlantic the beach disappeared in places and I was pushed up to the sand dunes. This became a problem where there was a high fence on the first dune, surrounding a golf course and I had very little room to get through between the waves breaking and splashing up the dune. I was glad to get through that section!
So now when I do the coastal route I keep a close eye on the weather forecast and also the tide times, for instance for Viana do Castelo.