Ok not many of you will be doing this actual trip but bear in mind if you are even put on the top deck of a ship/ferry.
I used the Trasmedditerania ship from Lanzarote to Cadiz on the night of 31st December needless to say it was dark.the ship was almost empty but myself along with roughly 30 other vehicles were parked on the top deck in the open air. The crossing 34hours was no drama and arrived in Cadiz at 06.30 and of course very dark.
I then travelled to Antiquera 200kms, and it was when arriving I noticed some black marks on the bodywork, I left it until the following day and realised the car was covered in these black marks which on closer inspection turned out to be soot from the ships funnel. The soot it turns out is very acidic and corrosive. The marks could not be washed off and one of the larger marks finally came off taking the paint with it down to the metal bodywork.
This has become a very big problem thankfully now resolved by Linea Directa who have been excellent.
The car being one year old has been in the workshop here in Antiquera stripped to the bones and totally resprayed plus many of the lights and bits of trim have been replaced. Costs over 6000€. And the shipping companies response was, did you report this on the morning of disembarking, take photos of the car, basically they did not want to know.
I have wondered since about a number of the other cars that were parked close to me in particular a brand new Mercedes and an Audi they must have been in the same position as myself, just be warned if you even get put on the top deck.