On the CP(VE) this year just after Easter, I would only book the night before at the most, sometimes on the day itself when I had a good idea where I might get to that afternoon. That worked quite well. I used a combination of booking.com and direct contact. The Xunta albergues on the Sanabres when I walked that in May were never more than half full. I booked a couple of places where there weren't Xunta albergues, but even they didn't seem to be full.
As a cautionary note, when I walked the CP in 2022, one albergue filled early in the evening, about 1800, and a mother and daughter were turned away shortly after that. The next albergue was over 6km further on, more that either felt comfortable walking at that time of the day. I always found it better to have a booking if I felt that I was going to arrive in the early evening.
There are a couple of different boats, and I think both have online booking, or you can go to the wharf or their booking office as well if you arrive early enough the previous afternoon. The smaller boat did fill up the day I took it. I don't know about the larger one.