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mob dog said..
............. It's designed for when your in and out of the water unlike a wetsuit which needs to be mostly submerged to keep you warm.
That is a total myth.
A wetsuit does NOT need to be 'mostly submerged' to work. The very best wetsuits are sealed to allow lttle or no water ingress at all. The drier they keep you, the better they keep ypu warm. It is the insulation of the closed cell neoprene foam that keeps you warm.

A spray jacket has almost no inherent insulation. It works by reducing chill factor caused by air movement and evaporation.
Loose fitting jackets are not ideal in the windsurfing situation where thay may trap water or catch on equipment. Better to have a more effective wetsuit.
One of the biggest things to keep warm is to cover and insulate you extremities. Hands (more difficult for windsurfing), Feet (easy) and especially your head with a neoprene hat or a helmet.