I’ve done the Santander ferry journey before. Personally, I’d never do it without a cabin. They are expensive but well worth it. Entertainment on-board can be fun, if a little cheesy.
I’ve looked, and my discount would only extend to a French crossing as it stands. If one of you wants to give me a call I can chat about it in more detail but there is a way you could save some more money here, I’m pretty sure.