I thought I would share my experience - since there were many unknowns. I booked a Grimaldi ferry from Barcelona to Porto Torres Sardinia. About 100 eu for travel and lodging for the night.
The ferry left at 11:59 pm. I booked online and they gave an address to pick up the tickets but it was not clear when you could get the tickets. I called their number repeatedly throughout the day and could never get through. So finally took a taxi to the address - which is a bit too far to walk - maybe a mile south of the area where the Ramblas meets the sea. It was a big building with other people showing up by taxi and big trucks arriving. Check in was smooth and was able to board with no line, or xray machine.
The room was really hard to find because they told you to go to a certain area and look for your room number, but among the 5-6 hallways none of them had a range that included my room number. It turns out you have to walk down one of the hallways, then jog over to another hallway, then continue down and you find your place. The employees were reluctantly helpful and slightly irritated by any questions, but mentioned that the rooms were in a certain general direction. Eventually found the room and noticed a lot of other passengers wandering around too.
The "keys" were made of paper like an old IBM punch card which did not seem to work at first. Several other passengers were standing outside their rooms messing with the keys too. Eventually we discovered the right touch to get the rooms to open. Pretty standard ferry rooms with an attached tiny bathroom.
Smooth journey all night - although with more rocking than a cruise ship. This ship is 250 m long and about 60,000 tons - one of their biggest. Got a fairly good sleep.
There were a total of 5 snackbar/restaurants. I think a double large cappuchino was 3.20 so not too bad.
The ship was arriving at 3 pm, but at 1 pm they announced to everyone to vacate their rooms and assemble on the restaurant deck so they could start cleaning for the next passengers soon to board. We arrived a bit early about 2:20 pm. The ship can hold 3000 passengers and I would guess they had 500-1000. A large number of passengers had a dog or dogs.
We arrived and there was a free shuttle bus waiting. I was expecting a mad rush, but after 25 minutes only 15 passengers got on the bus and we took off. It seems the vast majority of passengers had camper vans, motorcycles or cars.
We arrived at the Porto Torres Intermodal station. A fairly new cinderblock building that housed a (closed) tourist office, several ferry ticket offices, a bus office, and a little cafe with snacks. Bought the bus tickets for 3.50 or so, for an hour bus ride to Alghero. The buses arrive outside on the street. We were told we could also buy tickets on the bus, but it seemed to be better to buy in advance. It turns out it can be much more complicated but that is a story for another thread.