Or is it OK to just book when we are there in May?
We were in Porto end October last year and booked online the day before; time of year might matter though! We just booked based on tripadvisor reviews/comments. Seemed like lots of choice…if it wasn’t a nice weather day I wouldn’t bother!
I booked the same day last July and didn’t have any problem getting on a boat. I booked in the morning for an afternoon tour.