Hi Simon,
The wetsuit rule might be for safety reason.
If you crash and cannot relaunch or if the wind dies for any reasons, it might takes time for the rescue boats to come and get you. Your body temperature drops quickly when you stay still in the water. Having a wetsuit keeps your temperature high enough to wait for the rescue boat.
I remember doing a race in France where the wind dropped 2 km off shore. 150 racers waiting in the water with their kites. It took more than 1 hour for the 5 rescue boats to pickup everyone.
In this circumstance, go as quick as you can to the kite by pulling one front line only, then when you hold your kite safely, pack your lines as good as you can and wait on your kite for the safety boat to rescue you.
It should not happen, but always good to be prepared.