"Personal water craft" is a well established generic term for jet ski. Just hit google if you doubt it. If some moron from the Parks department wants to make his own definition because he has a dislike for sailboarders then take the fine, fight it in court and when the Parks department has a bunch of costs awarded against them maybe he will loose his job (unlikely).
If they want to ban windsurfers then they have to change the law. This country has had enough of trumped up people in uniform putting their own interpretation on laws (immigration dept etc). It is about time they got with the mood of the nation.
Even in Sa law PWC are defined as being power driven eg.
HARBORS AND NAVIGATION REGULATIONS 1994 - REG 8
8—Interpretation
"personal watercraft" means a device that—
(a) is propelled by a motor; and
(b) has a fully enclosed hull; and
(c) is designed not to retain water if capsized; and
(d) is designed to be operated by a person who sits astride, stands, or kneels on the device, and includes the device commonly referred to as a jet ski;
Take a copy of this Act with you with this definition when you go there again and wave it under their noses. And see whether they are willing to misuse their authority now they know they cant bluff you and you know THEY are acting illegally.
you can find it here.
www.austlii.edu.au/cgi-bin/sinodisp/au/legis/sa/consol_reg/hanr1994322/s8.html#personal_watercraft