if you were prepared you could wear a dry suit (like divers and boat racers) though its a little bulky (takes up a whole duffel bag)
immersion suits are not normally supplied for passenger use on large ships but they are supplied to crew, take a walk around the ship towards the bridge, engine room ect (areas where passengers are not allowed to normally enter) and you will start to come across immersion suits in large orange cases on the wall, rember where these are and nab one in an emergency if you can.
like wise on planes there is a large survival kit in a hatch in the ceiling directly above the emergency door over the wings
or you could carry one of these:
which is simmilar to what pilots use a personal mini life raft for if they have to eject over water
if however you were unprepared for such an event, then hopfully you will have memorised the routes to the life boats and got there at the FIRST sign of trouble. and you would have done you reseach and not have boarded a vessel which did not have adiquate emergency equipment.
failing that and you are on an ill equiped vessle which is sinking, gather anything which is boyant, preferably somthing large enough to keep you out of the water, put on warm clothing and look for a way of signaling
if you end up in the water bring your knees up to your chest and wrap your arms aroung them so thatyou roll up into a ball with your head out of the water, (the position reduces the surface area of your body in contact with the water) and do not try to swim as this would move the water around you removing the layer of water you just warmed up.