The heart of Rias Bajas is actually the ria of Pontevedra. I am quoting the tourist office in Vigo.
I was focusing on Vigo ria, which has many boats, but mainly going to Cies.
Around Pontevedra ria there are many cities with boats that do the mussels route. In Vigo there is only one that does it mainly on Sundays. So the offer is very scarce and it is often complete.
Around Pontevedra you can easily find a boat without reservation. Look at locations like Ogroove. As an example I called to a company and they were quite surprised when I asked to reserve.
Extra tip: I ended up doing a 1h15 cruise. Try to find one with at least half an hour more. It was really nice but you feel you needed more time to enjoy. Specially with the wine and the mussels served on board.