Just to play devils advocate here !!!
A PFD simply keeps you afloat ; which I`m pretty sure I do (most people??) in wetsuit and harness. (Try going under water in a wetsuit/harness ???)
Only time any flotation device would be of use is if you are knocked unconscious;in which case you are as likely to lay face down anyhow in a PFD??? A certified life jacket becomes the only viable help (ie one that floats you automatically on your back) I have never seen any windsurfer wearing such a device ???? (Not sure what the new numbers are but it used to be to BS 3595; PFD/Buoyancy aids do not meet standards)
IMO the best safety advice is never sail in conditions beyond your capabilities ; wear a helmet and suitable wetsuit/shoes and dont sail further out than you would be comfortable paddling your board back from?? Have a go at laying on board paddling. Its an essential self rescue skill.
I carry a length of rope and a spare UJ/Pin. (Bolts direct to fin box) If I`m sailing alone I take my mobile phone out (In aqua pack)
I`ve windurfed for 30 years and never been in a situation where a PFD would help ???? Had plenty of bruises on head; stood on urchins/rocks ???
Lets face it a board is a much better safety device than any PFD ????
How many instances have there been where PFD`s /Buoyancy aids have saved windsurfers ???
I dont know of any ?