I don’t know about now, but twice when I’ve had to cross there the ferry wasn’t running because the tide was low, but there was a man there with a small power boat that took me across for less than €10. He was docked near the ticket office for the ferry. The first time I used him I was told about his service by a woman inside that ran the little snack bar.
Everyone in Caminha knows Mario, the fisherman who ferries you across. Perhaps there are others, but if you ask at the tourist office, they will tell you how to reach Mario, so my advice is to go there first. His number, when we crossed, several years ago was (+351) 96-341-6259. Mario charged us 5 Euros for the crossing.
Just left Caminha this morning, ferry runs from 10 am, except for Mondays. There is a fisherman who can take you over the other side, but he has a small boat, not more than 6 people. He works from 7 am.