I am sure that without either a raft or a drysuit (or a wetsuit, but one isn't likely to be flying with one of those
) one would not survive for longer than an hour or two around here.
But... if you are in the water without a raft, how will they find you in a sea state other than calm, other than pure luck? Only your head/shoulders will stick out.
Unless I am missing something, I think a raft is very necessary.
Obviously one could get lucky and be seen by a boat or such.
Re the ditching course: I could be very wrong on this but I think the purpose of these "North Sea oil platform worker" ditching courses is to get out of a ditched helicopter which is upside down, but will not sink because it has floats; it just won't stay upright if the sea is too rough.
I am very sure that by the time a plane's cockpit is really filled with water (which for a non-pressurised aircraft means most of the water came in via the engine compartment and via the tail section which must also be full of water) the whole thing will sink rapidly to the bottom.
So one must get out very quickly. I am not suggesting a ditching course would be a waste of time; anything that brings people into rapid contact with cold water is worth doing - just that it isn't going to be representative of any survivable situation.