Brittany Ferries was mentioned earlier so I'll give a little of the info I know of two of them. The Cap Finisterre and Pont Aven run between Spain and the UK across the Bay of Biscay.
When you realise they are not counting the lower engine space etc, they are 10 decks high but only have a draught of 18 feet the mind boggles
Its not as though the trucks are on the lower decks either as there is not enough headroom, the lower two decks are for cars/ motorcycles. The next two decks are for trucks, motorhomes etc so even the weight is high in them meaning a higher centre of gravity.
For instance we have crossed on the Pont Aven in July when the 'Bay' has cut up rough, with stabilizers deployed the boat was still rolling so much it was impossible to stand without holding on to handrails etc- many passengers were sick that trip and the cleaners were everywhere with buckets and mops
I've attached a couple of detail shots, the first is the Pont Aven the others are the rear of the Cap Finistere. I have some overall shots, if I can find them I'll post them later.