Sony is the only one which seems to market their own waterproof housings, but there are companies which make other ones.
You can get something cheap and cheerful like this
http://www.gadget-bag.co.uk/product_details.php?id=1555
Up to things like these which are specific to a model and have controls on the outside and can be used underwater and cost a fortune..
http://www.marinevisions.com/catalog...bd7d01c9055a0b
When choosing a camcorder, I don't think there is anything specific to windsurfing that you need to look for - just get the best one you can afford. If you want standard definition, HDD, the SR90 is about as good as it gets (or arguably the JVC MG505). The comment about fast motion probably is referring to this being an issue for the MPEG2 compression - which all HDD cams use. But I don't see it really being a big issue.
You may want to add a polarizing filter to help with the glare on the water.