Had to hunt down some of my own footage for some fireworks. I wasn't near water so everything was pointed "up" and I turned up just as they were finishing.
I was shooting on a Red Scarlet X, 4KHD, ISO 800 and an f Stop of 2.8 to 4.0 on a 70-200mm Sigma. Also, some shots at 4KWS, at 48fps (or something around there). Bonus is the ISO adjust with the footage in post. I would say should at something around the 5.0 mark. Sometimes, if you get a massive bloom firework it can push the highlights way high.
My two cents.
Husavik-153.jpg by [url=https://www.flickr.com/photos/113899047@N06/]
J086_C013_0101EN by [url=https://www.flickr.com/photos/113899047@N06/]
J086_C012_0101JW by [url=https://www.flickr.com/photos/113899047@N06/]