The first birds,
an apparently moribund pair, turned up on 26th June. These birds were eating
welding slag after a recent mobilisation. They also engaged in
behaviour I have never witnessed before: the female appeared to feed
the male with some presumeably regurgitated food.
Another thing this pair did was sit with their nictating membrane (i.e.
"eyelid") shut. This may have been a response to the very strong
sunlight, glare from the sea and strong reflections from the white
superstructure of the vessel.

Female feeding
male Common Crossbill

One of the males that turned up the next day
Over the next several
days more Crossbills arrived onboard, usually singly or in pairs but on
one occasion a flock of seven were watched feeding in the netting of
the gangway which was stored on deck. The birds foraged
actively pulling out bits of fibre and other things from rope ends and
the like. Whether or not they found anything edible remains a mystery
(there were some insects onboard which could conceivably been
sheltering in amoung the ropes)
That Crossbills
don't just eat the seeds of pine cones is well documented but I have
never seen this level of desparation before.
Some birds did actually eat more normal stuff - crumbs fed to them by
crew members who were out sunbathing in the fantastic weather.
Feeding
in the rigging...