I'm not a vet but I get this question a lot when starting new dogs in flyball as well as dock diving. My response is always, "Your dog can get hurt just galavanting in your own backyard or at the park,its not always due to a sport". I have had one of our dog's most serious injuries happen just playing in the park.
Dead tail seems to be one of the most common injuries in dock diving and this is basically a strain of the muscles around the base of the tail. This can be caused by overexertion of the hind end. Some believe that cold water plays a part in this however I've seen non water dogs suffer from this ailment. Dead tail usually takes between a few days to a week to heal,soemtimes a little more. The vet will usually give you something like Rimadyl to help w/pain and inflamation. Proper warm ups/stretching, conditioning and limiting excessive repetitive stress on your dog's hind end are key factors in preventing dead tail.
As for torn ACLs or PCLs, these are usually caused by slips, slides, stepping into a hole when running, twisting, landing wrong, etc similar to the way a human would tear an ACL. Be careful in what you ask of your dog and the surface they run and play on.
I'm sure someone in the veterinary field could give you more detail...
Urs & the Crazy K9s
Gunner: PB:25', SV: 6'10" 2008 Top Border Collie, 2010 Top Veteran Dog & 2010 Frequent Flyer 1 Mile Achievement Award
Aries: PB: 27'8" , SV: 7' Co-SD Distance Record Holder, 2008, 2009 and 2010 Top Mixed Breed, 2009 and 2010 Extreme National Champion, 2009 and 2010 Top Dog Overall
Twiztid: PB: 10;06", 2008 Most Enthusiastic Team
Sniper: PB: 22'6", SV: 6'10"
Urs - The Boss Lady - 2007 Gabe Sportsmanship Award, 2009 Most Spirited Cheerleader