The answer is NO. While windsurfing is the main focus of your fitness program most of us cannot windsurf or do not windsurf often enough to improve our windsurfing fitness in a significant way. To improve your performance on your board you need a high level of overall fitness.
And the way to do this is to undertake a specific windsurfing fitness program to:
Develop the right muscles for windsurfing.
Decrease the risk of injury.
Maintain your fitness.
Weight training is also great for maintenance. As windsurfing is a seasonal sport you often get in shape during the windsurfing-season and most of the gains disappear through the off-season. Weights are a great way to maintain and even improve your fitness.