Information from my memory of The Making of Thunderball
, let me know if I got anything wrong and I will change it!
Thunderball is in fact the Bond movie that was originally a movie. Flemming was always keen to get the character onto the screen, and co-wrote Thunderball with another writer. He then wrote the novel, and was met with legal action. This delayed the production of Thunderball, so the first Eon film was Doctor No. The co-writer, Kevin McClory, retained enough rights to the story to make the almost identical Never Say Never Again.
Thunderball started production on the day Goldfinger opened in theatres, and they pulled off a grueling stint to get it out a year later. One of the biggest challenges facing the crew was the fact that a surprising amount of the movie takes place underwater. It is worth noting that the underwater propulsion devices in the movie were all made by some guy in Miami, and the designer said it gave him much confidence in his work, because "If I could think it up, there would be someone in the world who could make it"
Among other wonderful things that stand out is the Disco Volante, Emil Largo's yacht. In the movie it drops the back end and converts into a hydrofoil for a speedy getaway, and that really is what it does - they bought a hydrofoil and built a back end on it so it looks like a yacht. The joins consist of two couplings, and was pretty unstable, yet they managed to film on it for months. The final fight scene between Bond and Largo is not actually filmed on the yacht, but on a reconstruction. The Disco Volante met its end when the crew's liason with the US military (If you want to know his name watch Goldfinger, they put his name up as C.O. of Fort Knox) provided them with some rocket fuel for the climactic crash into a reef. They got it the night before filming, and did not get a chance to test it out, and so when the hydrofoil seems to leave suspiciously little wreckage after the crash that's because it's about a thousand feet in the air and decending FAST.
The sharks were all real, and all live, apart from the one that chases Sean Connery out of the pool. They had no idea how to handle live sharks, and learnt by trial and error. The scene in the underwater tunnel linking the pools where Bond meets Largo's sharks did not involve any acting, Connery really did freak out.... more than once actually. There is one bit where he is supposed to be in a perspex corridor, keeping the sharks out. Only no one told him there was a section missing, and guess where one of the sharks went at the first opportunity?
And if anyone tells you otherwise, you let them know that the jet pack is real.