I just got back from the Crystal River and snorkelling with the manatees. My only disappointment from the trip wasn't the awful vis at Kings but moreover came from a photographer at 3 sisters springs. We went there late in the morning after the first visit.............
Gary,
I'm glad you got some good viz at 3 sister's!!
Regarding King's Spring. Ahhhh if you had seen it even 3 years ago you wouldn't have complained at all!!! In fact if you ever get a chance to look on page 63 of 'Successful Underwater Photography", by Brian Skerry and Howard Hall, the manatee there I'm almost positive was photographed by Brian at King Spring. But probably 10-15 years ago. King Spring used to be the premier place for manatee as it was even more "consistent" and "dramatic" with the large tarpon, hordes of crevalle jacks, diving cormorants, etc., than the more volatile 3 Sister's Spring.
Briefly what happened there isn't completely man's fault but isn't completely nature's fault either. It is a complex combination. In 1993 a "no-name" storm hit the West Coast of Florida here in March. I remember it, most of my friends who live on Clearwater Beach woke up with 3 feet of water inside their houses, most of the cars ruined. The forecasters were really off on that storm. In Crystal River you could drive a pontoon boat on US 19 (where you probably stayed or ate lunch!).
What happened is it introduced so much salt water into the Crystal and Homosassa Rivers, much worse than any hurricane in recent memory, that it started a complicated process of killing most of the fresh water aquatic plants that used to keep the river water "crystal" clear. Then a Lyngbia sp algae (invasive), has taken over and has choked many remaining freshwater aquatic plants. But note the Crystal River used to be so clear that it has taken years for it to reach the point it is at now in 2007. South Florida Water Management District has been trying, since the mid 90s, to help clean up the Lyngbia sp invasive algae and bring back the other aquatic plants, but we may have lost the battle.
It is just heartbreaking for us to love the river, manatee, cormorant, fish, dolphin, etc etc etc, who make their home there. We can only hope King Spring may someday return to a more clear state, but it may never happen.
The manatee are fine, but even they have a harder time now finding enough vegetation to eat and have to go further from the springs to find food.
But we still have 3 sister's although that is not as clear as it used to be either and it is very volatile (good, clear days with manatee are rare). Also it can't support as many snorkelers (and photographers), so limits will be placed later this year on the numbers of people allowed in the water at Crystal and Homosassa Rivers by Fish and Wildlife. This is a good thing, for people and the manatee.
I was at 3 sister's last Thursday with several photographers and I'm happy to report that it was 95% polite and positive. Photographers there that day were very polite (I think most realize the situation with the river and appreciate that the good days there are so rare now they have to work together). Very positive day. And most important the manatee were not disturbed!
But I do acknowledge that "impolite" photographers and people can come and ruin things for everyone, but that is a fact of life that cannot be avoided anywhere you go - as you say it happens even more frequently on our highways!
I think we can only change things by trying to set as good an example ourselves as we can (granted we all make mistakes). And if something non-optimum happens try to correct it but if it is impossible just try, try again as they say. Persistence really pays off in photography and in all areas of life.
Good news is the manatee at Crystal River and Homosassa Rivers are doing pretty well. There are threats of course but people wanting to see them is the least of their threats for sure!
I'm sorry you encountered a rude photographer, but all are not like that. In fact I find that is the exception. Of course the best people make an extra effort and go early and on "uncrowded" days (as you did Gary), I've had plenty of days where I was the only person back in the springs.
I wish you had gone to 3 sister's first, but I think there might have been a problem with the low-tide water level (7amish I think) that day and that is why they decided to do 3 sister's last. Still you go to see manatee in clear water which is a rare thing these days!!! 
Thanks for alerting us about the rude photographer but I hope it didn't ruin your experience. Trust me "we" aren't all like that, I've even seen other photographers there help others get a shot!!! B)
All I've got to say for this thread is we all need to strive to be better in the water as I don't think anyone has an absolutely mistake-free record. Possibly by setting a good example ourselves and trying to effectively correct those that negatively effect the environment we can affect a change towards the better - and make our hearts smile when we get home and see the beautiful world we have photographed! 
Best, Carol