When you get cold the circulation to your fingers and toes, then your arms and legs are the first to be reduced. It's your bodies method of preventing freezing to death.
The energy to move your arms and legs comes from you blood so basically you ae cutting off the energy supply to the bits you want to use for windsurfing, so you get weak in the arms and legs get wobbly.
The blood also removes the by products of the energy production (lactic acid) so if you continue to stress your arms and legs with minimal blood flow, it builds up in the muscles resulting in cramps.
The best solution is to put the gear away for the winter.