This isn't meant to be a flame at all but a lot of the replies are way way of course to the point of madness.
The original question was what emergency kit to take on a RORO ferry in case of capsize in summer. Ask yourself what is the survival situation?
The first priority is you have to get off the ship before it sinks or pulls you down after it. Period. You don't stop and change into a drysuit or wetsuit, or attempt haul such kit through a dozen stairwells filled with hundreds of panicking people. You get out. As soon as you find your cabin check the escape routes and memorise them so you can negotiate them quickly and in the dark and when up is no longer up.
A large commercial ferry goes down. They can go down real fast. But they get off a mayday which includes location, or are missed pretty quickly. They are invariably on busy lanes and every ship in the vicinity will assist. Any period in the water is unlikely to be long term. Unless you are very close to shore the second priority should be to keep alive and in the same place and await rescue. Find a life raft and stay with other survivors. In a life raft you are a) more likely to be spotted by the rescue services, b) less likely to drown and c) going to stay warmer.
Being in the water for 24 hours in this scenario is pretty much worse case. You don't need emergency food or water, compasses (if you can't see land for f's sake stay where you are), binoculars, firesteels or knives. All of this will make you less likely to get off the ship in the first place and make treading water harder.
The #1 piece of kit would be, as posted already a GPS based Emergency Position-Indicating Radio Beacon (EPIRB) or Personal Locator Beacon (PLB).
The #2 piece of kit I would carry would be a small
emergency strobe. An orange smoke flare would be nice but consider that you are unlikely to get through customs with explosives (knives could be problematic also).
The #3 piece of kit would be a decent
life jacket. But they have to be able to turn you face up and you'd have to have hefty paranoia to buy one for a short trip and carry it around the rest of your holiday. A more practical solution would be to ensure you know where the ship issued life jackets are to grab on the way out.
Practical and functional clothing goes without saying but I wouldn't pack anything else.
No no no. Shock is a sudden drop in the blood flow to vital organs. When you are cold the external capillaries close down to direct blood to the body core and maintain vital organs. That's why fingers and toes go cold first. Alcohol makes you feel better but causes peripheral capillary dilation, drawing blood away from the core and worsening the effects of shock and hypothermia. Drinking spirits could kill you.
Re. shock: your body is used to the effects of gravity on your blood, and resisting it all pooling in your legs. When you are immersed in water the hydrostatic pressure counters the effects of gravity which results in an increased core blood volume. Your body counters this by telling your kidneys to process more urine. You get a similar effect when you lay down and go to sleep and so always need to pee in the morning even though you've not been drinking. The problem is that the body will stabilize the core blood volume to account for the hydrostatic pressure, and then you will be rescued, plucked from the water, lose the hydrostatic pressure, blood flows back to the legs and BAM: that is when you do get major shock. There were a lot of rescued divers killed like this before it was figured out by the RAF, they would be fine in the water and dead by the time the winch got them to the helicopter. These days they winch rescued divers on stretchers to stop it happening but I don't know what it is like in other countries and if you are plucked out by a passing fisherman they probably won't know so expect shock and keep horizontal with legs raised.