Ok, hear me out... so you're walking north along the Portuguese coast, coming from Vila Praia de Ancora. At Moledo, you have the option to turn right and go under the train tracks, then to Caminha. Or you can go straight through Moledo, walk through a beautiful beachside forest with a well-marked, well-travelled trail, and come out onto the beach at the tip of the Foz do Minho.
On your right, there's a water taxi stand for Capitao Mario. They speak no English, just point to their sign saying "6 euros." You give them cash and the silent Capitao Mario takes you right away onto a speedboat painted like a shark. He hands you a lifejacket but doesn't insist that you put it on. He rushes you three minutes at high speed across the river, and rams the boat up onto the beach in Spain. You hop off and there's a boardwalk right there on the beach, which you can follow into A Guarda. A nice, flat walk with beautiful views, and also shortly after you land you can make a stop at the Hoteles El Molino. There's even a change station and foot-washing station at the beach there if you want to take a dip in the ocean before heading into A Guarda.
If you take the official route and go to Caminha instead, you may have to wait for the boat to depart at a specific time, and then you also face the choice of going over the hill and down into A Guarda, or taking the boardwalk with its beautiful views and flat path into A Guarda (but it's 3 extra km from Camposancos compared to going over the hill).
If you go straight at Moledo and take Capitao Mario's boat, you have no extra distance (because you didn't have to go to Caminha), a fantastic boat experience, delightful views and beach, and it's a flat walk to A Guarda.