Producer William Alland was indeed at a party at Orson Welles' house in the 1940's and Mexican cinematographer Gabriel Figueroa told everyone a story about a "Man Fish" in the Amazon. This story stuck with Mr. Alland until he was able to sell the idea to Universal as a good story for a monster movie. This would have been around about 1952. Alland was producer for all three movies.
The underwater sequences were indeed filmed in Florida. Most of the underwater work representing the Black Lagoon was done at Wakulla Springs with some additional footage shot at Silver Springs for the second as that was closer to Marineland just north of Daytona Beach where most of the filming for the second movie took place. Other Florida locations were the St. Johns river, Jacksonville, especially the Lobster House (now a brewery) and Fort Myers.
Ricou Browning played the underwater creature in all three movies and also created the TV show Flipper, did work on the Sea Hunt TV, and worked as second unit director, stunt coordinator and underwater sequence director on a number of features, including Thunderball and Never Say Never Again.
Ben Chapman played the on land creature in the first movie and since then was a tireless champion of the Creature and the creature movies until his passing in 2008 (his family now maintains his website). Check it out at http://www.the-reelgillman.com/
Tom Hennesy played the on land creature in the second movie and Don McGowan played this role in the third movie.
While Bud Westmore got credit for the makeup on the creature movies the people who really did the work and brought the creature that we know and love to life are as follows.
Millicent Patrick: Did the artistic design which means she is the most responsible for the image of the Gillman.
Chris Mueller: Sculpted the head.
Jack Kevan: Sculpted the rest of the body.
So you could say Chris and Jack brought Millicent's designs to life.
Jack Arnold was the overall director for the first two movies with John Sherwood taking the reigns for the third movie. For the first movie the underwater work was supervised by James C. Havens, and Charles (Scotty) Welbourne actually was in the water doing the filming.
All of this information was taken from the documentary "Back to the Black Lagoon" by film historian David J. Skal and the audio commentary available on the DVD's of the three movies with film historian Tom Weaver (all three movies), film historian Bob Burns (second and third movies) and the female co-star of the second movie Lori Nelson (second movie).
This is not meant to be an all inclusive history of these movies and the people involved so if you want more, buy the Creature from the Black Lagoon Legacy Collection and watch the documentary and listen to the audio commentary and do some research of your own on the internet and in film history books. It's a fascinating subject and I'm sure you'll enjoy yourself.
Links:
http://www.floridastateparks.org/wakullasprings/
http://www.jaxhistory.com/Jacksonville Story/Picture of Creature from Black Lagoon.htm