Post from a Windsurfer
As someone with a bit of knowledge about this (ruptured my own & studied the theory behind ruptures a bit) I'd say that windsurfers should not worry too much about rupturing the achilles.
Put very short, it is extremely difficult to rupture an achilles by impact alone; alternatively from "outside" pull. Pretty much the only thing that can make it happen is when your muscles in your entire leg puts as much pressure on it as possible; something that pretty much only happens in dynamic sports such as basketball (what I did to mine), ultimate, badminton etc., where quick and aggressive changes of direction happen. At very few situations (if any) will there be anywhere near such stress on the achilles when windsurfing; generally so much less that even a blow to the tendon itself should not rupture it. Falling with feet in straps would be a lot more dangerous to the rest of the ankle as well as knee ligaments than the achilles.
Btw. conservative treatment (i.e. no surgery, regrowin while in cast) seems to show good results while avoiding the dangers of surgery (very little blood circulation in the schilles = very high/bad effects of a potential infection after surgery).
I personally am not doing any real dynamic sports yet and probably wont for another year (to avoid rerupture) but I have been windsurfing (carefully) since little more than two and a half month afte the rupture and full on four months after; and have not felt that I was vulnerable when windsurfing; just slower/less precise on one tack due to less strength/coordination (getting there now).
I worry about pretty much anything else (bone breaks, ancle and knee ligaments, head-to-board/rig etc.) before worrying about my achilles when windsurfing. And yeah, the injury stinks but with enough determination most people wil be back faster than from a lot of other injuries sustainable while suring (like ACL rupture or bad ancle breaks). Hope this helps?