I don't think it's going to be a tri, because you can't have as many people do it. They are ratcheting the marathon weekend races over 20,000 runners; Princess had about 15,000 registered. Even the big triathlons are usually no more than 2000 or 3000 people, with a few exceptions (I think Chicago is a really big one).
Swimming in Bay Lake is not really that bad, but the water is very dark because of the tannins. And weedy in places. And smells of boat fuel. The 1/2 Ironman has always been in Bay Lake, Danskin is now at Bay Lake, but used to be in the Seven Seas Lagoon. The amoeba thing is mainly a problem if you get water way up your nose...regular swimming shouldn't be a huge issue, but there is still some danger there. Also, most of the swim is out in the deeper waters so less an issue for that.
And like Matt said...I've swum in much skankier and warmer water.
Wonder why there wasn't a clue...oh, well...